Despite being demonstrably better than conventional surgical techniques with regards to implant alignment and outlier reduction, computer navigation systems have not faced widespread adoption in surgical operating rooms. We believe that one of the reasons for the low uptake stems from the bulky design of the optical tracker assemblies. These trackers must be rigidly fixed to a patient's bone and they occupy a significant portion of the surgical workspace, which makes them difficult to use. In this study we introduce the design for a new optical tracker system, and subsequently we evaluate the tracker's performance. The novel tracker consists of a set of low-profile flexible pins that can be placed into a rigid body and individually deflect without greatly affecting the pose estimation. By relying on a pin's stiff axial direction while neglecting lateral deviations, we can gain sufficient constraint over the underlying body. We used an unscented Kalman filter based algorithm as a recursive body pose estimator that can account for relative marker displacements. We assessed our tracker's performance through a series of simulations and experiments inspired by a total knee arthroplasty. We found that the flexible tracker performs comparably to conventional trackers with regards to accuracy and precision, with tracking errors under 0.3mm for typical operating conditions. The tracking error remained below 0.5mm during pin deflections of up to 40mm. Our algorithm ran at computation speeds greater than real-time at 30Hz which means that it would be suitable for use in real-time applications. We conclude that this flexible pin concept provides sufficient accuracy to be used as a replacement for rigid trackers in applications where its lower profile, its reduced invasiveness and its robustness to deflection are desirable characteristics.

The hip centre (HC) in Computer Assisted Orthopedic Surgery (CAOS) can be determined either with anatomical (AA) or functional approaches (FA). AA is considered as the reference while FA compute the hip centre of rotation (CoR). Four main FA can be used in CAOS: the Gammage, Halvorsen, pivot, and least-moving point (LMP) methods. The goal of this paper is to evaluate and compare with an in-vitro experiment (a) the four main FA for the HC determination, and (b) the impact on the HKA.

The experiment has been performed on six cadavers. A CAOS software application has been developed for the acquisitions of (a) the hip rotation motion, (b) the anatomical HC, and (c) the HKA angle. Two studies have been defined allowing (a) the evaluation of the precision and the accuracy of the four FA with respect to the AA, and (b) the impact on the HKA angle.

For the pivot, LMP, Gammage and Halvorsen methods respectively: (1) the maximum precision reach 14.2, 22.8, 111.4 and 132.5 mm; (2) the maximum accuracy reach 23.6, 40.7, 176.6 and 130.3 mm; (3) the maximum error of the frontal HKA is 2.5°, 3.7°, 12.7° and 13.3°; and (4) the maximum error of the sagittal HKA is 2.3°, 4.3°, 5.9°, 6.1°.

The pivot method is the most precise and accurate approach for the HC localisation and the HKA computation.

INTRODUCTION

The functional and anatomical results of TKA revisions are less good than a primary TKA. The TKA revision frequency increases and we must improve our surgeries and prepare the next standard of these surgeries. The aim of this study was to evaluate the CAOS / one stage strategie to treat the knee PJIs.

Introduction

Accurate alignment of components in total knee arthroplasty (TKA) is a known factor that contributes to improvement of post-operative kinematics and survivorship of the prosthetic joint. Recently, CAOS has been introduced into TKA in effort to reduce positioning variability that may deviate from the mechanical axis. However, literature suggests that clinical outcomes following TKA with CAOS may not present a significant improvement from traditional methods of implantation. This would infer that achieving correct alignment, alone, might be insufficient for ensuring an optimal reconstruction of the joint. Therefore, this study seeks to evaluate the importance of soft-tissue balancing, through the quantification of joint kinetics collected with intraoperative sensors, with or without the combined use of CAOS.

A key determinant of long-term implant survival following primary TKA is post- operative alignment of the limb and components. The aim of this study was to compare the accuracy of the Vector-Vision CT-free navigation system versus conventional hand-guided TKA by comparing post-operative alignment.

In a retrospective study 51 sets of post-operative radiographs were analysed, 33 computer-guided and 18 hand-guided. A specific protocol for the measurement of post-operative TKA radiographs was outlined and a novel Trigonometric Method (TM) of angle measurement was compared with the traditional Goniometer Method (GM) of measurement.

The standardised protocol was applied to all 51 sets of radiographs. In total, six angles were measured on each radiograph by two independent observers and compared between the computer-guided and hand-guided groups.

A protocol for the measurement of post-operative TKA radiographs was delineated with step-by-step instructions. The TM of angle measurement had a precision of 1.06° compared with 1.5° using the GM. The standard deviation of the TM was significantly smaller than the GM (p=0.033) and the intra-class correlation coefficient (ICC) of the TM was 0.94 versus 0.90 for the GM.

For the Mechanical Axis (MA), 91% of patients in the computer-guided group attained a MA within 180±3o compared with only 78% in the hand-guided group. T he absolute median raw deviation from 180° was 0.8 in the navigated group and 1.9° in the hand-guided group (p=0.029). Thus, the navigated group was associated with significantly less variability about the neutral 180°. For the other five angle measurements, a higher percentage of patients attained a more neutral alignment with computer-guided TKA; however, these did not reach statistical significance

The computer-assisted group demonstrated significantly more neutral alignment following TKA, and this may in turn lead to reduced TKA failure rates and improved implant longevity. In addition, a new TM of angle measurement was found to be more superior in terms of precision in comparison to the traditional method.

Computer-assisted orthopaedic surgery (CAOS) improves mechanical alignment and the accuracy of surgical cuts in the context of total knee arthroplasty. A simplified, CAOS enhanced instrumentation system was assessed to determine if the same effects could be achieved through the use of a less intrusive system. Two cohorts of surgeons (experienced and trainees) performed a series of total knee arthroplasty resections in knee models with and without navigation-enhanced instrumentation. The percentage of resections that deviated from the planned cut by more than 2°or 2mm (outliers) was determined by post-resection advanced imaging for six unique outcome metrics. Within each experience level, the use of the CAOS enhanced system significantly reduced the total percentage of outliers as compared to conventional instrumentation (Figure 1). The experienced users improved from 35% to 4% outliers overall (p < .001) and the trainees from 34% to 10% outliers (p < .001). Comparing across experience levels, the experienced surgeons performed significantly better in only a single resection metric with conventional instrumentation (Figure 2A), varus/valgus tibial alignment, with 8.3% outliers compared to the trainee's 63% outliers (p = .004). The use of CAOS enhanced instrumentation eliminated any differences between the two user groups for all measured resections (Figure 2B). Comparing CAOS enhanced to conventional instrumentation specifically between anatomical deformity types revealed that there is significant improvement (p < .05) with the use of enhanced instrumentation for all three deformity types (Figure 3). These results suggest that non-intrusive CAOS enhanced instrumentation is a viable alternative to conventional instrumentation with possible benefits. This trial also demonstrates that additional experience may not correlate to improved surgical accuracy, and outliers may be less a result of individual surgeon ability or specific anatomic deformities, and more so related to limitations of the instrumentation used or other yet unidentified factors

As previous meta-analyses on the alignment outcomes of Computer-assisted orthopaedic surgery (CAOS) did not differentiate between CAOS systems, limited information is available on the accuracy of a specific CAOS system based on clinical cases. This study assessed the accuracy and precision of achieving surgical goals in approximately 7000 cases using a specific contemporary CAOS system. Alignment parameters were extracted from the technical logs of 6888 TKA surgeries performed between October 2012 and January 2017 using a contemporary CAOS system. The following surgical parameters were investigated: 1) planned resection defined by the surgeon prior to the bone cuts; 2) Checked resection defined as digitalisation of the bony cuts. Deviations in alignment between planned and checked resections were evaluated, with acceptable resections defined as no more than 3° of resection deviations. For the tibial resection, deviations in tibial varus/valgus angle and posterior tibial slope were 0.06 ± 0.94° and −0.09 ± 1.73°, respectively. For the femoral resection, deviations in femoral varus/valgus angle amd femoral flexion were 0.00 ± 0.97° and −0.17 ± 1.44°, respectively. High percentages of the resections were found to be acceptable (>94% of the cases). The CAOS system investigated was shown to provide accurate and precise intra-operative assistance to the surgeon in achieving targeted resections. The study summarised a large number of cases spanning the application history of the specific CAOS system, including both experienced users and new adopters of the technology. The data provided a complete clinical relevant evaluation demonstrating its high accuracy and precision in resection alignment

Computer-assisted orthopaedic surgery (CAOS) has been demonstrated to increase accuracy to component alignment of total knee arthroplasty compared to conventional techniques. The purpose of this study was to assess if learning affects resection alignment using a specific CAOS system. Nine surgeons, each with >80 TKA experience using a contemporary CAOS system were selected. Prior to the study, six surgeons had already experienced with CAOS TKA (experienced), while the rest three were new to the technology (novice). The following surgical parameters were investigated: 1) planned resection, resection parameters defined by the surgeon prior to the bone cuts; 2) checked resection, digitalisation of the realised resection surfaces. Deviations in the alignment between planned and checked resections were compared between the first 20 cases (in learning curve) and the last 20 cases (well past learning curve) within each surgeon. Any significance detected (p < 0.05) with >1° difference in means indicated clinically meaningful impact on alignment by the learning phase. Both pooled and surgeon-specific analysis exhibited no clinically meaningful significant difference between the first 20 and the last 20 cases from both experienced and novice surgeon groups. The resections in both the first 20 and the last 20 cases demonstrated acceptable rates of over 95% in alignment (<3° deviation) for both experienced and novice surgeons. This study demonstrated that independent of the surgeon's prior CAOS experiences, the CAOS system investigated can provide an accurate and precise solution to assist in achieving surgical resection goals with no clinically meaningful compromise in alignment accuracy and outliers during the learning phase

INTRODUCTION. Despite that computer-assisted orthopaedic surgery (CAOS) has been shown to offer increased accuracy to the bony resections compared to the conventional techniques [1], previous studies of CAOS have mostly focused on alignment outcomes based on a small number of patients [1]. Although several recent meta-analyses on the CAOS outcomes have been reported [2], these analyses did not differentiate between systems, while system-dependency has been reported to influence alignment parameters [3]. To date, no study has benchmarked a specific CAOS system based on a large number of clinical cases. The purpose of this study is to assess the accuracy and precision of bony resection in more than 4000 cases using a specific contemporary CAOS system. Materials and Methods. Technical logs of 4292 TKAs performed between October 2012 and January 2016 using a contemporary CAOS system (ExactechGPS, Blue-Ortho, Grenoble, FR) were analyzed. The analyses were performed on: 1) planned resection, defined by the surgeon prior to the bone cuts. These parameters serve as inputs for the CAOS guidance; and 2) Checked resection, defined as digitalization of the actual resection surfaces by manually pressing an instrumented checker onto the bony cuts. Deviations in alignment and resection depths (on the referenced side) between planned and checked resections were calculated in coronal and sagittal planes for both tibia and femur (planned vs checked). Results. Summary and distribution of the deviations in resection parameters are presented in Table 1 and Fig 1. On average, the alignment deviations were near 0°, and the deviations in resection depths were less than 0.15mm. Small standard deviations were observed. Discussion. This study demonstrated that the CAOS system investigated can offer accurate and precise intra-operative guidance to the surgeon in achieving his/her surgical goals. TKA performed using conventional instruments is reported to achieve satisfactory lower limb alignment (within ±3° of alignment deviation) in only 70–80% of the cases [2,4], which may contribute to 20% of patients being dissatisfied with the results of surgery [5]. This study reviewed a large number of cases spanning the application history of the specific CAOS system, providing a complete clinically relevant evaluation of its accuracy and precision in terms of bony resection. The results confirmed that the system investigated can be used with confidence that the surgical goals can be achieved with accuracy and reliability. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

Total knee arthroplasty (TKA) is an effective technique to treat end-stage knee osteoarthritis, targeting the restore a physiological knee kinematics. However, studies have shown abnormal knee kinematics after TKA which may lead to suboptimal clinical outcomes. Posterior slope of the tibial component may significantly impact the knee kinematics. There is currently no consensus about the most appropriate slope. The goal of the present study was to analyse the impact of different prosthetic slopes on the kinematics of a PCL-preserving TKA, with the hypothesis that posterior slopes can alter the knee kinematics. A PCL-retaining TKA (Optetrak CR, Exactech, Gainesville, FL) was performed by a board-certified orthopaedic surgeon on one fresh frozen cadaver that had a non arthritic knee with an intact PCL. Intact knee kinematic was assessed using a computer-assisted orthopaedic surgery (CAOS) system (ExactechGPS®, Blue-Ortho, Grenoble, FR) Then, TKA components were implanted using the guidance of the CAOS system. The implanted tibial baseplate was specially designed to allow modifying the posterior slope without repeatedly removing/assembling the tibial insert with varying posterior slopes, avoiding potential damages to the soft-tissue envelope. Knee kinematic was evaluated by performing a passive range of motion 3 separate times at each of the 4 posterior slopes: 10°, 7°, 4° and 1°, and recorded by the navigation system. Femorotibial rotation, antero-posterior (AP) translation and hip-knee-ankle (HKA) angle were plotted with regard to the knee flexion angle. Tibial slopes of 1° and 4° significantly altered the normal rotational kinematics. Tibial slopes of 7° and 10° led to a kinematics close to the original native knee. All tibial slopes significantly altered the changes in HKA before 90° of knee flexion, without significant difference between the different slopes tested. The magnitude of change was small. There was no significant change in the AP kinematics between native knee and all tested tibial slopes. Changing the tibial slope significantly impacted the TKA kinematics. However, in the implant studied, only the rotational kinematics were significantly impacted by the change in tibial slope. Tibial slopes of 7° and 10° led rotational kinematics that were closest to that of a normal knee. Alterations in knee kinematics related to changing tibial slope may be related to a change in the PCL strain. However, these results must be confirmed by other tests involving more specimens

INTRODUCTION. Studies have reported that only 70–80% of the total knee arthroplasty (TKA) cases using conventional instruments can achieve satisfactory alignment (within ±3° of the mechanical axis). Computer-assisted orthopaedic surgery (CAOS) has been shown to offer increased accuracy and precision to the bony resections compared to conventional techniques [1]. As the early adopters champion the technology, reservation may exist among new CAOS users regarding the ability of achieving the same results. The purpose of this study was to investigate if there are immediate benefits in the accuracy and precision of achieving surgical goals for the novice surgeons, as compared to the experienced surgeons, by using a contemporary CAOS system. Materials and Methods. Two groups of surgeons were randomly selected from TKAs between October 2012 and January 2016 using a CAOS system (ExactechGPS, Blue-Ortho, Grenoble, FR), including:. Novice group (7 surgeons): no navigation experience prior to the adoption of the system and have performed ≤20 CAOS TKAs. To investigate the intra-group variation, this group was further divided into surgeons with extensive experience in conventional TKA (novice-senior), and surgeons who were less experienced (novice-junior). Experiences group (6 surgeons): used the CAOS system for more than 150 TKAs. All the surgeries from the novice group (86 cases) and the most recent 20 cases from each surgeon in the experienced group (120 cases) were studied. Deviations in the resection parameters between the following were investigated for both tibia and femur: 1) planned resection, resection goals defined prior to the bone cuts; 2) checked resection, digitization of the realized bone cuts. The deviations were compared within the novice group (novice-senior vs novice-junior), as well as between the novice and experience groups. Knees with optimal resection (deviation<2°/mm, without clinically alter the joint mechanics [2]) and acceptable resection (deviation<3°/mm, as commonly adopted) were identified. Significance was defined as p<0.05. Results. A summary of the deviations is presented in Table 1. No statistical differences were found between the senior and the junior surgeons in the novice group. Similarly, no differences were found between the experienced group and novice group, except for that the cases in the novice group tended to resect slightly more bone in the tibia (p<0.01), and had slightly larger standard deviations compared to the experienced group. The experienced and novice groups had comparable, high percentages of the knees in both the optimal and acceptable categories (Fig 1). Discussion. This study demonstrated that regardless of the surgeon's experience with TKA, new adoption of the CAOS system investigated can immediately benefit the accuracy and precision of the bony resections at a comparable level with experience CAOS users. Although significant difference was found between novice and experienced groups in tibial resection depth, the difference (0.57mm) was clinically irrelevant. The CAOS system offers substantial reduction of the outliers compared to TKAs performed with conventional instruments [3]

Introduction. Computer-assisted orthopaedic surgery (CAOS) has been shown to assist in achieving accurate and reproducible prosthesis position and alignment during total knee arthroplasty (TKA) [1]. The most prevalent modality of navigator tracking is optical tacking, which relies on clear line-of-sight (visibility) between the localizer and the instrumented trackers attached to the patient. During surgery, the trackers may not always be optimally positioned and orientated, sometimes forcing the surgeon to move the patient's leg or adjust the camera in order to maintain tracker visibility. Limited information is known about tracker visibility under clinical settings. This study quantified the rotational limits of the trackers in a contemporary CAOS system for maintaining visibility across the surgical field. Materials and Methods. A CAOS system (ExactechGPS®, Blue-Ortho, Grenoble, FR) was set up in an operating room by a standard surgical table according to the manufacture's recommendation. A grid with 10×10 cm sized cells was placed at the quadrant of the surgical table associated with the TKA surgical field [Fig. 1A,B]. The localizer was set up to aim at the center of the grid. A TKA surgical procedure was then initiated using the CAOS system. Once the trackers-localizer connection was established, the CAOS system constantly monitored the root mean square error (RMS) of each tracker. The connection was immediately aborted if the measured RMS was above the defined threshold. Therefore, “visibility” was defined as the tracker-localizer connection with proper accuracy level. An F tracker from the tracker set (3 trackers with similar characteristics) was placed at the center of each cell by a custom fixture, facing along the +Y axis [Fig. 1]. The minimum and maximum angles of rotation around the Z axis (RAZ_MIN and RAZ_MAX) and X axis (RAX_MIN and RAX_MAX) for maintaining tracker visibility were identified. For each cell, the rotational limit of the tracker was calculated for each axis of rotation as the difference between the maximum and minimum angles (RLX and RLZ). Results. The tracker rotation limits were 144.7±3.9° for RLZ (range: 136°–152°), and 150.5±3.9° for RLX (range: 143°–158°). RLX was significantly higher than RLZ across the field (difference in means=5.8°, p<0.01). Along the X axis, the rotational limit decreased slightly for RLZ, but increased slightly for RLX [Fig. 2]. Discussion. Studies have pointed out that the need for maintaining line-of-sight can be a limitation for the use of optical tracking based CAOS systems [2,3]. The results here demonstrated that ExactechGPS provides tracker visibility for more than 135° rotation across the surgical field. Moreover, the system is placed inside the sterile field, eliminating the potential blockage of the optical localizer by the surgical staff, further ensuring tracker visibility. The slight rotational limits trends along the X axis may be due to camera placement at one side of the surgical table. The current methodology may be applied to other CAOS systems to quantify the tracker visibility in a clinical environment. To view tables/figures, please contact authors directly

Introduction. While total knee arthroplasty (TKA) improves postoperative function and relieves pain in the majority of patients with end stage osteoarthritis, its ability to restore normal knee kinematics is debated. Cadaveric studies using computer-assisted orthopaedic surgery (CAOS) system [1] are one of the most commonly used methods in the assessment of post-TKA knee kinematics. Commonly, these studies are performed with an open arthrotomy; which may impact the knee kinematics. The purpose of this cadaveric study was to compare the knee kinematics before and after (open or closed) arthrotomy. Materials and Methods. Kinematics of seven non-arthritic, fresh-frozen cadaveric knees (PCL presumably intact) was evaluated using a custom software application in an image-free CAOS system (ExactechGPS, Blue-Ortho, Grenoble, FR). Prior to the surgical incision, one tracker was attached to the diaphysis of each tibia and femur. Native intact knee kinematics was then assessed by performing passive range of motion (ROM) three separate times, from full extension to at least 110 degrees of flexion, with the CAOS system measuring and recording anatomical values, including flexion angle, internal-external (IE) rotation and anterior-posterior (AP) translation of the tibia relatively to the femur, and the hip-knee-ankle (HKA) angle. Next, an anterior incision with a medial parapatellar arthrotomy was performed, followed by acquisition of the anatomical landmarks used for establishing an anatomical coordinate system in which all the anatomical values were evaluated [2]. The passive ROM test was then repeated with closed and then open arthrotomy (patella manually maintained in the trochlea groove). The anatomical values before and after knee arthrotomy were compared over the range of knee flexion using the native knee values as the baseline. Results. Generally, kinematics from the native knee were found to be similar to those with closed and open arthrotomy. Deviations between native knee and arthrotomy groups (open or closed, whichever was the worst case) were 0.49±0.52mm for the AP translation, 0.44±0.41° for the HKA, and 0.86±0.8° for the IE rotation (Figures 1–3). The deviation from native knee kinematics was found to be higher with increased flexion angles in both HKA and AP translation. Closing the arthrotomy had minimal effect on knee kinematics, and no difference was seen in knee kinematics between an open and closed arthrotomy, so long as the patella is manually maintained within the trochlear groove. Discussion. This study demonstrated arthrotomy, whether open or closed, did not affect the tested knee kinematics compared to a native intact knee. The deviation found in the anatomical values was within the typical range of clinical variation. Increased deviation in high flexion for some anatomical values may be due to difficulty in reproducing consistent motion during ROM test. This study showed that an open arthrotomy with the patella maintained in the trochlea groove provides accurate assessment of the intact knee kinematics

INTRODUCTION. Although several meta-analyses have been performed on total knee arthroplasty (TKA) using computer-assisted orthopaedic surgery (CAOS) [1], understanding the inter-site variations of the surgical profiles may improve the interpretation of the results. Moreover, information on the global variations of how TKA is performed may benefit the development of CAOS systems that can better address geographic-specific operative needs. With increased application of CAOS [2], surgeon preferences collected globally offers unprecedented opportunity to advance geographic-specific knowledge in TKA. The purpose of this study was to investigate geographic variations in the application of a contemporary CAOS system in TKA. Materials and Methods. Technical records on more than 4000 CAOS TKAs (ExactechGPS, Blue-Ortho, Grenoble, FR) between October 2012 and January 2016 were retrospectively reviewed. A total of 682 personalized surgical profiles, set up based on surgeon's preferences, were reviewed. These profiles encompass an extensive set of surgical parameters including the number of steps to be navigated, the sequence of the surgical steps, the definition of the anatomical references, and the parameters associated with the targeted cuts. The profiles were compared between four geographic regions: United States (US), Europe (EU), Asia (AS), and Australia (AU) for cruciate-retaining (CR) and posterior-stabilized (PS) designs. Clinically relevant statistical differences (CRSD, defined as significant differences in means ≥1°/mm) were identified (significance defined as p<0.05). Results. For resection parameters, CRSDs were found between regions in posterior tibial slope (PTS), tibial resection depth, as well as femoral flexion for both CR and PS profiles (marked in Table 1). Regarding anatomical references, US was the only region using posterior cruciate ligament (PCL) as the reference for CR resection depth (Table 1). Differences in percentage of preference were found in the anatomical references for tibial varus/valgus, tibial resection depth, femoral varus/valgus, femoral axial rotation, and ankle center (Table 1,2). For surgical steps, EU and AU were found to apply gap balancing technique as a common practice for the PS designs, while for the CR designs, EU and AU considerably adopted this technique (Table 2). For PS designs, EU and AU profiles preferred tibial first in the resection workflow, compared to a more balanced preference for other regions. For CR designs, US profiles were in favour of performing the femoral resection first in the workflow, compared to a strong favouring of tibial first resection workflow in EU and AS Am regions. Discussion. This study demonstrated clinically significant geographic differences may exist in the surgeons' preference of surgical parameters, anatomical references, and surgical workflow steps during TKA. These differences may reflect the geographic variations of surgeon training, surgical philosophy, or the specific characteristics of the patient population, which warrants further investigation. The strength of this study was that it is the first study to date that covered all the available surgical profiles spanning the application history of a specific CAOS system. As such, variation due to the operational differences of multiple systems was avoided. For any figures or tables, please contact authors directly (see Info & Metrics tab above).

Introduction. An emerging consensus in the surgical specialties is that skill acquisition should be more emphasized during surgical training. 1. This study was an attempt to evaluate the effects of repetitive practices using an image-free computer-assisted orthopaedic surgery (CAOS) guidance system (Exactech GPS, Blue-Ortho, Grenoble, FR) on both technical and cognitive skills. Materials and methods. A senior knee replacement surgeon with limited previous experience with the CAOS system performed a series of consecutive simulated knee surgeries using a commercially available artificial leg (MITA trainer leg M-00058, Medical Models, Bristol, UK). In order to assess the effects repetitive practice has on technical skills, we evaluated two indexes:. Error index: A unitless indication of overall error magnitude obtained by averaging the absolute values of all linear and angular measurement differences between targeted and checked cuts. Time index: An indication of the time required to acquire landmarks, adjust the custom blocks, and make cuts. In order to assess the effect repetitive practice has on cognitive skills, we evaluated the number of times the surgeon elected to deviate from pre-surgical planning or re-acquire landmarks. We evaluated these parameters for three chronological and consecutive groups of simulated surgeries: Group A (knee models #1 to #10), Group B (knee models #11 to #20), and Group C (knee models #21 to #28). Results. Regardless of the number of operations, tibial and femoral cuts were associated with a low error index (ranging from 0.45 to 0.71 for all three groups), suggesting the continuous guidance offered by the CAOS system provided an opportunity to correct discrepancies from the plan during surgery (Figure 1A). The variability of surgical time at key steps substantially reduced from Group A to Group C (Figure 1B). Finally, the surgeon elected to re-perform femoral landmark acquisitions 8 times for Group A, 4 times for Group B, and 0 times for Group C. Discussion. The authors attempted to delineate the effects of repetitive practices on skills using a CAOS system. The overall perception was the number of sequential practice surgeries had no significant effect on surgical accuracy. The significant decrease in the time index (43%) during the course of the practice surgeries is in line with recent studies regarding the learning curve associated with navigation for knee arthroplasty. 2. Compared to the initial planning, the only modifications in surgical steps were related to re-acquiring femoral landmarks. As with any image-free system, the present CAOS system relies on precise landmark acquisition. To achieve this goal, the system under consideration presents an interactive software enabling landmark visualization (Figure 2) allowing immediate feedback loop. Despite the obvious limitation of being conducted on synthetic bones, this study enabled a senior surgeon to perfect his technical and cognitive skills, potentially leading to increases in efficiency and efficacy in the intense environment of the operating room

Introduction. Clinical outcomes for total knee arthroplasty (TKA) are especially sensitive to lower extremity alignment and implant positioning. 1. The use of computer-assisted orthopaedic surgery (CAOS) can improve overall TKA accuracy. 2. This study assessed the accuracy of an image-free CAOS guidance system (Exactech GPS, Blue-Ortho, Grenoble, FR) in both a synthetic leg with a normal mechanical axis and legs with abnormal mechanical axis. Materials and methods. A high-resolution 3D scanner (Comet L3D, Steinbichler, Plymouth, MI) was used to scan varus-deformed (n=12), neutral (n=12), and valgus-deformed (n=4) knee inserts (Mita M-00566, M-00598, M-00567; respectively, Medical Models, Bristol, UK) and collect pre-identified anatomical landmarks prior to using the models to simulate knee surgery. The image-free CAOS guidance system was then used to acquire the same landmarks. After adjusting the position and orientation of the cutting block to match the targets, bone resections were performed, and the knee models were re-scanned. The 3D scans made before and after the cuts were overlaid and the resection parameters calculated using the pre-identified anatomical landmark data and advanced software (UG NX, Siemens PLM, Plano, TX). Data sets obtained from the 3D scanner (see Figure 1A) were compared with data sets from the guidance system (see Figure 1B). Given the accuracy of the 3D scanner (<50μm), its measurements were used as the baseline for assessing CAOS system error. Results. Table I shows errors in bone resection thickness orientation measurement errors as well as CAOS system confidence intervals (CI) for both the tibia and femur, depending on deformity type. Regardless of knee deformity and other parameters, the mean error of the CAOS system was systematically less than 0.5 mm for bone resection measurements and 1° for joint angle measurements. The 95% CI were in the range of −1.54 to 0.67mm for bone resection measurements and −0.64° to 1.67° for joint angle measurements. No statistical differences were detected between different deformity groups in the Error Indexes for both the tibia and femur. Discussion. This study represents an extension of a previous evaluation of the same CAOS system, where only a limited number of neutral models (n=6) were investigated. The current study was performed to reassess the accuracy and precision of the CAOS system using the same methodology with a larger number of knee models (n=28) exhibiting different types of deformities affecting the mechanical axis. In conclusion, this study demonstrates a high level of in-vitro accuracy for the CAOS system, regardless of leg-alignment deformity type. The mean error of the CAOS system, characterized as the difference between the measured and checked values, was systematically less than 0.5 mm for bone resection measurements and 1° for joint angle measurements

Introduction. Evaluations of Computer-assisted orthopaedic surgery (CAOS) systems generally overlooked the intrinsic accuracy of the systems themselves, and have been largely focused on the final implant position and alignment in the reconstructed knee [1]. Although accuracy at the system-level has been assessed [2], the study method was system-specific, required a custom test bench, and the results were clinically irrelevant. As such, clinical interpolation/comparison of the results across CAOS systems or multiple studies is challenging. This study quantified and compared the system-level accuracy in the intraoperative measurements of resection alignment between two CAOS systems. Materials and Methods. Computer-assisted TKAs were performed on 10 neutral leg assemblies (MITA knee insert and trainer leg, Medial Models, Bristol, UK) using System I (5 legs, ExactechGPS®, Blue-Ortho, Grenoble, FR) and System II (5 legs, globally established manufacturer). The surgeries referenced a set of pre-defined anatomical landmarks on the inserts (small dimples). Post bone cut, the alignment parameters were collected by the CAOS systems (CAOS measured alignment). The pre- and post- operative leg surfaces were scanned, digitized, and registered (Comet L3D, Steinbichler, Plymouth, MI, USA; Geomagic, Lakewood, CO, USA; and Unigraphics NX version 7.5, Siemens PLM Software, Plano, TX, USA). The alignment parameters were measured virtually, referencing the same pre-defined anatomical landmarks (baseline). The signed and unsigned measurement errors between the baseline and CAOS measured alignment were compared between the two CAOS systems (significance defined as p<0.05), representing the magnitude of measurement errors and bias of the measurement error generated by the CAOS systems, respectively. Results. The measurement errors are presented [Table 1]. For unsigned measurement error, System II was higher in the tibial varus/valgus alignment and posterior slope (p≤0.01), and lower in the femoral varus/valgus alignment (p=0.03), compared to System I [Fig. 1]. System II exhibited higher error bias towards tibial varus alignment (up to 2.59°), more posterior slope (up to 1.41°), and more femoral hyper extension (up to 1.6°) than System I (p<0.01) [Fig. 1]. The mean signed and unsigned errors were generally less than 1°, except for System II in the measurement of tibial varus/valgus alignment (signed and unsigned mean errors=1.93°). Discussion. This study reported system-dependent bias and variability associated with intraoperative measurements of alignment parameters during TKA. The results showed that System I generally had lower variability and less bias than System II. Although the majority of the significant differences found were clinically irrelevant (<1° in means), System II was notably shown to produce on average ∼2° measurement errors in tibial varus/valgus alignment biased towards varus. Intra-operative measurement of surgical resection parameters during imageless computer-assisted TKA surgery is a critical step, in which a surgeon directly relies on the real-time data to prepare the bony resections and check the final realized cuts. Clinical-level accuracy in alignment outcomes has been shown to be system-dependent [2], this study further suggested there are differences in system-level accuracy between CAOS systems

Introduction. Computer-assisted orthopaedic surgery (CAOS) has been shown to help achieve accurate, reliable and reproducible prosthesis position and alignment during total knee arthroplasty (TKA) [1]. A typical procedure involves inputting target resection parameters at the beginning of the surgery and measuring the achieved resection after bone cuts. Across CAOS systems, software/hardware design, mechanical instrumentation, and system-dependent work flow may vary, potentially affecting the intraoperative measurement of the achieved resection. This study assessed the cumulative effect of system-dependent differences between two CAOS systems by comparing the alignment deviation between the measurement of the achieved resection and the targeted parameters. Materials and Methods. TKA resections were performed on 10 neutral whole leg assemblies (MITA knee insert and trainer leg, Medial Models, Bristol, UK) by a board-certified orthopaedic surgeon (BH) using System I (5 legs, ExactechGPS®, Blue-Ortho, Grenoble, FR) and System II (5 legs, globally established manufacturer). The surgeon was deemed as “experienced” user (>30 surgeries) with both systems. The target parameters for the TKA resections, as well as major differences between the two systems are summarized in Table 1A. The deviations of the intraoperative alignment measurements on the achieved distal femoral and proximal tibial resection from the target were calculated and compared between the two systems with significance defined as p<0.05. Results. The alignment deviations (signed and unsigned) are presented in Table 1B. On average, System II had significantly higher deviation towards varus (2.2°) than System I (0.83° valgus) for the tibia (p<0.01) [Table 1B]. System I tended to measure slightly more in flexion (∼1°) than System II (∼0.5° extension) (p=0.03). System I demonstrated lower variability of the signed deviation (SD) than System II in tibial varus/valgus alignment, femoral flexion/extension, and femoral varus/valgus alignment [Fig. 1]. No significant differences were found in between systems in the unsigned errors. Both systems had measurement within the perceived acceptable range (within 3°) [2,3]. Discussion. Intraoperative measurement of the achieved TKA resections is important as it allows for intraoperative adjustment if the resections are not deemed suitable. Assuming a consistent surgical variability exhibited by the same surgeon with equal experience on both systems, this study demonstrated that some systems (System II) may have higher variability than others (System I), and exhibit clinically meaningful bias (tibial varus/valgus) while achieving or quantifying the resections. The variability may be caused by the cumulated effect of the differences between the two systems [see Table 1A]. As clinical alignment accuracy has been found to be system-dependent in a previous study [4], and archived resection parameters in the surgical report has been used as key inputs in relevant studies [5], the results here emphasizes the importance of taking into account the specific CAOS system in both clinical application and CAOS research. To view tables/figures, please contact authors directly

Introduction. Computer-assisted orthopaedic surgery (CAOS) provides great value in ensuring accurate, reliable and reproducible total knee arthroplasty (TKA) outcomes [1,2]. Depending on surgeon preferences or patient factors (e.g. BMI, ligament condition, and individual joint anatomy), resection planning (guided adjustment of cutting blocks) is performed with different knee flexion, abduction/adduction (ABD/ADD) and internal/external (I/E) rotation angles, potentially leading to measurement errors in the planned resections due to a modified tracker/localizer spatial relationship. This study assessed the variation in the intraoperative measurement of the planned resection due to leg manipulation during TKA, and identified the leg position variables (flexion, ABD/ADD, and I/E rotation) contributing to the variability. Materials and Methods. Computer-assisted TKA (ExactechGPS®, Blue-Ortho, Grenoble, FR) was performed on a neutral whole leg assembly (MITA knee insert and trainer leg, Medial Models, Bristol, UK) by a board-certified orthopaedic surgeon (BH) at his preferred leg flexion, ABD/ADD, and I/E rotation angles. A cutting block was adjusted and fixed to the tibia, targeting the resection parameters listed in Table 1A. An instrumented resection checker was then attached to the cutting block to measure the planned resection at the same leg position (baseline). Next, the surgeon moved the leg to 9 sampled positions, representing typical leg position/orientation associated with different steps during TKA [Table 1B]. The planned resection was tracked by the CAOS system at each leg position. Tibial resection parameters at each sampled position were compared to the baseline. Regression was performed to identify the variables (flexion, ABD/ADD, I/E rotation) that significantly contribute to the measured variation (p<0.05). Results. The resection parameters at the baseline leg position are presented (see Table 1A). Clinically negligible variations were found across the 9 positions [Table 1B], with mean errors ≤0.1mm in resection depths and ≤0.2° in alignment parameters. For this particular system analyzed, leg flexion strongly correlated with the measurement errors in medial resection depths (p≤0.01, R2=0.76), lateral resection depth (p=0.01, R2=0.61) and posterior slope (p<0.01, R2=0.92) [Fig. 1]. The system tended to measure less in resection depths and posterior slope with an increased leg flexion [Fig. 1]. No other statistical significance was found (N.S.). Discussion. The results here showed that ExactechGPS can provide robust measurements of the planned resection parameters during TKA, independent of the ABD/ADD and I/E rotation of the knee. Although for the system studied, measurement errors strongly correlated with leg flexion, the magnitude of the errors was clinically negligible (within ±0.5 mm/° at a confidence level of 95%) [Table 1B]. Although CAOS systems have been evaluated for accuracy in the spatial distance measurement and clinical alignment outcomes [2,3], the measurement accuracy of planned resection parameters due to change of leg position remains unknown, even though it directly impacts the final resection. This study provided an improved understanding of clinical variability on the measurement of planned TKA resection when using a CAOS system

Introduction. From pre-operative planning to final implant cementation, total knee arthroplasty (TKA) preparation is a succession of many individual steps, each presenting potential sources of error that can result in devices being implanted outside the targeted range of alignment. This study assessed alignment discrepancy occurring during different TKA steps using an image-free computer-assisted orthopaedic surgery (CAOS) guidance system (Exactech GPS, Blue-Ortho, Grenoble, FR) in normal and abnormal mechanical axis. Materials and methods. We used a commercially available artificial leg (MITA trainer leg M-00058, Medical Models, Bristol, UK) able to receive (neutral / varus / valgus) knee inserts simulating the proximal tibia and distal femur. A pre-surgical profile was established to define resection parameters for the proximal tibial and distal femoral cuts (Figure 1A). Data from the guidance system were collected at three separate steps: (1) cutting block adjusted but not pinned to the bone (Figure 1B), (2) cutting block adjusted and pinned to the bone (Figure 1C), and (3) after the cuts were checked (Figure 1D). These data were then compared to the resection target parameters to track potential dispersions occurring during the process. Due to the amount of data (i.e., four studied resection parameters per bone, three operative steps, and three knee model types), the authors introduced an “error index”, which was a unitless indication of overall error magnitude obtained by averaging the absolute values of all linear and angular measurement errors. Due to knee model dimensions (∼55 mm), the authors equally considered linear and angular measurement values (i.e., 1 mm equivalent to 1°). Results. Regardless of resection parameter or bone deformity type, all linear or angular error distributions were symmetrical around the neutral value, which implies no obvious skew in terms of error direction. The type of knee model deformity had almost no effect on overall error magnitudes throughout all surgical steps (Figure 2). Discussion. Few studies present possible causes for errors when using CAOS for TKA. Notably, Bathis et al. evaluated cutting errors as the difference between the primary cutting block position and the resulting resection plane. As a result, errors due to a malpositioning of the guide jig itself were not described. 1. In general, the authors found the dispersions at each step to seemingly be random. For both the tibia and the femur, a significant increase in the error index from the adjusted to the attached step (p<0.001 and p=0.005; respectively) was observed, meaning the pinning of the cutting block to the bone is a key step. Also, observing the relationship between linear and angular parameters was relevant. For example, for the femur, a cut in extension was highly correlated with lower than expected distal femoral resection (Pearson correlation factor of 0.783 and 0.913 at the checked step for the medial and lateral distal femoral resections; respectively, p<0.001). Regardless of the presence and type of deformity, the evaluated image-free computer-assisted guidance system did not exhibit substantial alignment dispersions during any step of the procedure