Controversy still exists in the literature regarding efficacy and usefulness of CASN in knee arthroplasty. However, obsession with basic alignments and proper correction of mechanical axes fails to recognise the full future potential of CASN which seems to lie in enhanced dynamic assessment. Basic dynamics usually at least includes intraoperative assessment of limb alignments, flexion-extension gap balancing and simple testing through ranges of motion. However our upgraded CASN system (Brainlab) is also capable of enhanced assessment not only including the provision of data on initial to final alignments but also contact point observations. The system can also perform an enhanced ‘Range Of Motion’ (ROM) analysis including observation of epicondylar axis motion, valgus and varus, antero-posterior shifts as well as flexion and extension gaps. Tracking values for both tibiofemoral and patellofemoral motion have also been obtained after performing registration of the prosthetic trochlea.

Observations were then made using a set of standardised dynamic tests. Firstly, the lower leg was placed in neutral alignment and the knee put through a flexionextension cycle. Secondly the test was repeated but with the lower leg being placed into varus and internal rotation. The third test was performed with the lower leg in valgus and external rotation.

We have been able to carry out these observations in a limited case series of 15 total knee arthroplasties and have found it possible to observe and quantify marked intra-operative variation in the stability characteristics of the implanted joints before corrections have been made and final assessments performed. Indeed contact point observation has found several cases of edge loading before corrections have been made. Also ROM analysis has demonstrated the ability of the system in other cases to observe and then make necessary adjustments of implant positions and ligament balance which alter the amounts of antero-posterior and lateral translations. In this way paradoxical antero-posterior and larger rotational movements have been minimised. Cases where conversion to posterior stabilisation has been necessary have been encountered. Also patellar tracking has been observed during such dynamic tests and appropriate adjustments made to components and soft tissue balancing.

Although numbers in this case series are small, it has been possible to begin to observe, classify and quantify patterns of instability intra-operatively using simple stress tests. Such enhanced intra-operative information may in future make it possible to create algorithms for logical adjustments to ligament balance, component sizes, types and positions. In this way CASN becomes a more useful tool.

Differing descriptions of patellar motion relative to the femur have resulted from many in-vitro and in-vivo studies. The aim of this study was to examine the tracking behaviour of the patella. We hypothesized that patellar kinematics would correlate to the trochlear geometry and that differing previous descriptions could be reconciled by accounting for differing alignments of measurement axes.

Seven normal fresh-frozen knees were CT scanned and their kinematics with quadriceps loading was measured by an optical tracker system and calculated in relation to the previously-established femoral axes. CT scans were used to reliably define frames of reference for the femur, tibia and the patella. A novel trochlear axis was defined, between the centres of best-fit medial and lateral trochlear articular surfaces spheres.

The path of the centre of the patella was circular and uniplanar (RMS error 0.3mm) above 16°±3° knee flexion. The distal end of the median ridge of the patella entered the groove at 6° knee flexion, and the midpoint at 22°. This circle was aligned 6.4° ± 1.6° (mean± SD) from the femoral anatomical axis, 91.2°±3.4° from the epicondylar axis, and 88.3°±3° from the trochlear axis, in the coronal plane. In the transverse plane it was 91.2°±3.4° and 88.3°±3° from the epicondylar and trochlear axes. Manipulation of the data to different axis alignments showed that differing previously-published data could be reconciled. When the anatomic axis of the femur was used to align the coordinates, there was an initial medial and then a lateral translation. Comparing this with the uniplanar and circular path of the center of the patella, it shows that the orientation of the femoral coordinate system affects the description of the patellar medial-lateral translation.

This study has shown the effect of using different coordinate systems on reporting the patellar translation. Choosing a femoral reference that is more in line with the plane of the circular path of motion and the trochlear groove in the coronal plane diminishes the reported subsequent lateral translation of the patella. Once the frame of reference had been aligned to the trochlear axis, there was minimum medial-lateral translation of the patella.

Introduction: Patellofemoral complications are one of the major causes for revision surgery. In the prosthetic knee, the main determinant within the patellofemoral mechanism is said to be the design of the groove (Kulkarni et al., 2000). Other studies characterising the native trochlear groove used indirect methods such as photography, plain radiographs and measurements using probes and micrometer. The aim of this study was to define the 3-dimensional geometry of the femoral trochlear groove. We used CT scans to describe the geometry of the trochlear groove and its relationship to the tibiofemoral joint in terms of angles and distances.

Materials and Methods: CT scans of 45 normal femurs were analysed using custom designed imaging software. This enabled us to convert the scans to 3D and measure distances and angles. The flexion axis of the tibiofemoral joint was found to be a line connecting the centres of the spheres fitted to posterior femoral condyles. These two centres and the femoral head centre form a frame of reference for reproducible femoral alignment. The trochlear geometry was defined by fitting circles to cross sectional images and spheres to 3D surfaces. Axes were constructed through these centres. The deepest points on the trochlear groove were identified using quad images and Hounsfield units. After aligning the femur using different axes, the location of the groove was examined in relation to the mid plane between the centres of flexion of the condyles.

Results: The deepest points on the trochlear groove can be fitted to a circle with a radius of 23mm (S.D. 4mm) and an R.M.S error of 0.3mm. The groove is positioned laterally (especially in its mid portion) in relation to the femoral mechanical and anatomical axes. It was also lateral to the perpendicular bisect of the transcondylar axes. After aligning the anatomical axis in screen the trochlear groove can be described on average to be linear with less than 2 mm medial/lateral translation.

In the sagital view, the centre of the circle is offset by 21mm (S.D.3mm) at an angle of 67° (S.D. 7°) from a line connecting the midpoint between the centres of the femoral condyles and the femoral head centre.

On either end of this line, the articular surface of the trochlea can be fitted to spheres of radius 30mm (S.D. 6mm) laterally and 27mm (S.D. 5mm) medially, with an rms of 0.4mm.

Discussion: The location and configuration of the inter-condylar groove of the distal femur is clinically significant in the mechanics and pathomechanics of the patellofemoral articulation. This investigation has allowed us to characterise the trochlear groove.

This can be of use in planning and performing joint reconstruction and have implications for the design of patello-femoral replacements and the rules governing their position.

Objective: The aim of the study was to test the hypothesis that insertion of a total knee replacement (TKR) may effect range of motion as a consequence of excessive stretching of the retinaculae.

Methods: 8 fresh frozen cadaver knees were placed on a customised testing rig. The femur was rigidly fixed allowing the tibia to move freely through an arc of flexion. The quadriceps were loaded to 175N in their physiologic lines of action using a cable, pulley and weight system. The iliotibial tract was loaded with 30N. Tibiofemoral flexion and extension was measured using an optical tracking system. Monofilament sutures were passed along the fibres of the medial patellofemoral ligament (MPFL) and the deep transverse band in the lateral retinaculum with the anterior ends attached to the patella. The posterior suture ends were attached to ‘Linear Variable Displacement Transducers’. Thus small changes in ligament length were recorded by the transducers. Ligament length changes were recorded every 10° from 90° to 0° during an extension cycle. A transpatellar approach was used when performing the TKR to preserve the medial and lateral retinaculae. Testing was conducted on an intact knee and following insertion of a cruciate retaining TKR (Genesis II). Statistical analysis was performed using a two way ANOVA test.

Results: The MPFL had a mean behaviour close to isometric, while the lateral retinaculum slackened by a mean of 6mm as the knee extended from 60 degrees (Fig 1). After knee replacement there was no statistically significant difference seen in ligament length change patterns in the MPFL, however the lateral retinaculum showed significant slackening from 10 to 0°.

Conclusion: The data does not support the hypothesis that insertion of a TKR causes abnormal stretching of the retinaculuae. This result relates specifically to the TKR design tested.

Objective: The aim of this study was to test the hypothesis that malrotation of the femoral component following total knee replacement (TKR) may lead to patellofemoral complications as a consequence of excessive stretching of the retinaculae.

Methods: 8 fresh frozen cadaver knees were placed on a customised testing rig. The femur was rigidly fixed allowing the tibia to move freely through an arc of flexion. The quadriceps and iliotibial tract were loaded to 205N in their physiologic lines of action using a cable, pulley and weight system. Tibiofemoral flexion and extension was measured using an optical tracking system. Monofilament sutures were passed along the fibres of the medial patellofemoral ligament (MPFL) and the deep transverse band in the lateral retinaculum with the anterior ends attached to the patella. The posterior suture ends were attached to ‘Linear Variable Displacement Transducers’. Thus small changes in ligament length were recorded by the transducers. Ligament length changes were recorded every 10° from 90° to 0° during an extension cycle. A transpatellar approach was used when performing the TKR to preserve the medial and lateral retinaculae. Testing was conducted following insertion of a cruciate retaining TKR (Genesis II). The femoral component was rotated using a custom built intramedullary device. Ligament length changes were measured at neutral rotation, 5° internal and 5° external rotation. Statistical analysis was performed using a two way ANOVA test.

Results: Internal rotation resulted in the MPFL slackening a mean of 1.7mm from 70-0° extension (p< 0.001). External rotation resulted in the MPFL tightening a mean of 1.5mm over the same range (p< 0.01). The lateral retinaculum showed less significant differences.

Conclusion: External rotation resulted in smaller length changes than internal rotation. Patellar tilting as a result of internal rotation may be caused by MPFL slackening and not lateral retinacular tension, contrary to popular understanding.

Objective: This study tested the hypothesis that complications resulting from overstuffing the patellofemoral joint after total knee replacement (TKR) may be a consequence of excessive stretching of the retinaculae.

Methods: 8 fresh frozen cadaver knees were placed on a customised testing rig. The femur was rigidly fixed and the tibia moved freely through an arc of flexion. The quadriceps and iliotibial tract were physiologically loaded to 205N using a cable, pulley and weight system. Tibiofemoral flexion/extension was measured using an optical tracking system. Monofilament sutures were passed along the fibres of the medial patellofemoral ligament (MPFL) and the deep transverse band in the lateral retinaculum with the anterior ends attached to the patella. The posterior suture ends were attached to ‘Linear Variable Displacement Transducers’. Thus, small changes in ligament length were recorded by the transducers. Length changes were recorded every 10° from 90°- 0° during an extension cycle. A transpatellar approach was used when performing the TKR to preserve the medial and lateral retinaculae. Testing was conducted following insertion of a cruciate retaining TKR (Genesis II). The patella was resurfaced and various patellar thicknesses were achieved by placing 2mm thick nylon washers behind the ‘onlay’ button. The thicknesses measured were 2mm understuff, pre-cut thickness, 2 and 4mm overstuff. Statistical analysis was performed using a two way ANOVA test.

Results: Patellar understuff resulted in the MPFL slackening an average of 1.6mm from 60 to 0° (p< 0.05). Overstuffing the patella 2mm resulted in no significant length changes whereas 4mm overstuff resulted in a mean increase in MPFL length of 2.3mm throughout extension (p< 0.001). No significant length changes seen in the lateral retinaculum

Conclusion: Overstuffing the PFJ stretches the MPFL, because it attaches directly between two bones. The lateral retinaculum attaches to the relatively mobile ITT, so overstuffing does not stretch it.

Patellofemoral complications in total knee arthroplasty (TKA) are common. Patellar tracking can be adversely affected by component positioning, soft tissue imbalance and bony deformity. Lateral patellar release rates reported in the literature vary from 6– 40%. Computer assisted surgery has largely been confined to the tibio-femoral component of total knee replacement. However, with recently developed software, it can be used to visualise and quantify patellar tracking and thus guide the precise extent and site of lateral patellar release. The aim of this early study was to define the diagnostic envelope for identification and quantisation of patella maltracking using a current generation patella navigation system.

Our previous prospective analysis of 100 patients undergoing primary TKA identified pre-operative radiographic indices that correlate with maltracking of the patellofemoral joint. 20 cases were subsequently selected for computer assisted total knee replacement surgery. The navigation system (Vector Vision (BrainLab) version 1.6) was used to achieve accurate alignment and position of the femoral and tibial components. All knee replacements were performed using a posterior cruciate-retaining prosthesis. The femoral component was of a ‘patella-friendly’ design with inbuilt 3 degrees external rotation, and the patella was resurfaced in all cases with a biconvex inlay patellar prosthesis.

Patellar tracking was assessed intra-operatively using an additional patellar array and patella tracking-specific software. Real-time displays of patella shift, tilt, rotation and circle radii during multiple flexion-extension cycles were obtained. Where necessary, an ‘outside-to-in’ release of the lateral retinacular complex was performed. The navigation system was used to provide contemporaneous feedback on the effect of the soft tissue releases on the tracking characteristics of the patella component on the prosthetic trochlea. Primary outcomes included the sensitivity and specificity of the system for peri-operative patella maltracking; secondary outcomes included the definition of interventional endpoints and correlation of intra-operative tracking data with post-operative x-rays.

The demographic data for the 20 patients enrolled in this study was essentially unremarkable. As compared to standard intra-operative clinical evaluation of patella tracking, the computer navigation system is equally sensitive and specific, and it can potentially detect more subtle instances of maltracking that may elude conventional clinical evaluation. We present patterns of patellar tracking during the surgery for patient with and without pre-operative patellar maltracking. However, the significance of this is unknown without longer-term outcome data. Patella shift abnormalities that were detected by the system, but not tilt, correlated with clinical judgement of patella maltracking (p< 0.05).

Soft tissue balancing of the patella can now be performed by observing precise changes in shift and tilt. This can be as important as component alignment for optimising patellar tracking and minimising patellofemoral complications.