This study measured the three bony axes usually used for femoral component rotation in total knee arthroplasty and compared the accuracy and repeatability of different measurement techniques.

Fresh cadaveric limbs (n=6) were used. Three observers (student, trainee and consultant) identified the posterior condylar (PCA), anteroposterior (AP) and the transepicondylar (TEA) axes, using a computer navigation system to record measurements. The AP axis was measured before and after being identified with an ink line. The TEA was measured by palpation of the epicondyles both before and after an incision was made in the medial and lateral gutters at the level of the epicondyles, allowing the index finger to be passed behind the gutters. In addition the true TEA was identified after dissection of all the soft tissues. Each measurement was repeated three times. For all axes and each observer the repeatability coefficient was calculated.

The identification of the PCA was the most reliable (repeatability coefficient: 1.1°) followed by the AP after drawing the ink line (4.5°) then the AP before (5.7°) and lastly the TEA (12.3°) which showed no improvement with the incisions (13.0°). In general the inter-observer variability for each axis was small (average 3.3°, range 0.4° to 6°), being best for the consultant and worst for the student. In comparison to the true TEA, the recorded TEA and AP axis averaged within 1.5° whilst the PCA was consistently 2.8° or more internally rotated.

This study echoed previous studies in demonstrating that palpating the PCA intra-operatively is highly precise but was prone to errors in representing the true TEA if there was asymmetrical condylar erosion. The TEA was highly variable irrespective of observer ability and experience. The line perpendicular line to the AP axis most closely paralleled the true TEA when measured after being identified with an ink line.

Previous work has demonstrated vulnerability of the femoral nerve to damage by anterior acetabular retractors during THA. The aim of this study was to quantify the proximity of the femoral nerve to the anterior acetabulum, on cadaveric material and MRI studies.

A standard posterior approach to the hip was carried out in 6 fresh frozen cadaveric hemipelves. Following dislocation and removal of the femoral head, measurements were taken from the anterior acetabular lip to the posterior aspect of the femoral nerve as it passed over this point. 14 MRI studies of the hip were obtained from the local PACS database (7 male, 7 female; mean age 58 (range 32–80)). T1 weighted axial scans were reviewed. Measurements were obtained from the anterior acetabular lip to the posterior surface of the femoral nerve and artery, and the cross-sectional area of iliopsoas was calculated.

There was no significant difference between the mean distances to the femoral nerve in the cadaveric (24 mm) and MRI groups (25.3mm) (p=0.7). On MRI images, the distance between the acetabular wall and both the femoral artery (p=0.003) and femoral nerve (p=0.007) was significantly larger in men. The femoral artery is strikingly close to the acetabulum in females, passing a mean distance of 14.8 mm, whereas in males this was 23.9 mm. The mean femoral nerve distance was 28.7 mm in males and 21.9 mm in females. The cross-sectional area of iliopsoas was significantly smaller in women (5.97 cm² compared to 11.37 cm², p<0.001).

Both the femoral artery and nerve run in close proximity to the anterior acetabular lip. Care should be taken when placing instruments in this area to avoid neurovascular injury. The increased incidence of femoral nerve damage in women following THA may be due to the significantly smaller bulk of iliopsoas.

Acetabular retractors have been implicated in damage to the femoral and obturator nerves during total hip arthroplasty (THA). Despite this association, the anatomical relationship between retractor and nerve has not been elucidated.

A posterior approach to the hip was carried out in 6 fresh frozen cadaveric hemi- pelvises. Large Hohmann acetabular retractors were placed anteriorly over the acetabular rim, and inferiorly, as per routine practice in THA. The femoral and obturator nerves were identified through dissection and their relationship to the retractors was examined.

If contact with bone was not maintained during retractor placement, the tip of the anterior retractor had the potential to compress the femoral nerve, by passing either superficial to, or through the bulk of the iliopsoas muscle. If pressure was removed from the anterior retractor, the tip pivoted on the anterior acetabular lip, and passed superficial to iliopsoas, overlying and compressing the femoral nerve, when pressure was reapplied. The inferior retractor pierced the obturator membrane, medial to the obturator foramen in all specimens. Subsequent retraction resulted in the tip moving laterally to contact the obturator nerve.

Both the femoral and obturator nerves are vulnerable to injury around the acetabulum through the routine placement of retractors in THA. The femoral nerve is vulnerable where it passes over the anterior acetabulum. Iliopsoas can only offer protection if the retractor passes deep to the muscle bulk. If pressure is removed from the anterior retractor intra-operatively it should be reinserted. The obturator nerve is vulnerable as it exits the pelvis through the obturator foramen. Vigorous movement of the inferior retractor should be avoided. Awareness of the anatomy around the acetabulum is essential when placing retractors.

Local infiltration analgesia is a relatively novel technique developed for effective pain control following total knee replacement, reducing requirements of epidural or parenteral post-operative analgesia. The study aimed to investigate the anatomical spread of Local Infiltration Analgesia (LIA) used intra-operatively in total knee arthroplasty (TKA) and identify the nerve structures reached by the injected fluid.

Six fresh-frozen cadaveric lower limbs were injected with 180ml of a solution of latex and India ink to enable visualisation. Injections were done according to our standardised LIA technique. Wounds were closed and limbs were placed flat in a freezer at −20°C for two weeks. Limbs were then either sliced or dissected to identify solution locations.

Injected solution was found from the proximal thigh to the middle of the lower leg. The main areas of concentration were the popliteal fossa, the anterior aspect of the femur and the subcutaneous tissue of the anterior aspect of the knee. There was less solution in the lower popliteal fossa. The solution was found to reach the majority of the terminal branches of the tibial, fibular and obturator nerves.

Overall, there was good infiltration of nerves supplying the knee. The lack of infiltration into the lower popliteal fossa suggests more fluid or a different injection point could be used. The solution that travelled distally to the extensor muscles of the lower leg probably has no beneficial analgesic effect for a TKA patient. This LIA technique reached most nerves that innervate the knee joint which supports the positive clinical results from this LIA technique. However, there may be scope to optimise the injection sites.

Introduction

Soft tissue balancing is an important aspect of total knee replacement surgery. Traditionally sequential medial soft tissue release is performed for balancing in varus deformity. Its effects on kinematics and dynamic Femoro-Tibial-Mechanical-Angle (FTMA) have been described in extension and 90° flexion in coronal plane. However most studies have missed what happens when the knee flexes from 0 to 90 degrees This study is one of the first to describe its effects on knee kinematics throughout flexion. The aim was to look at deviation of FTMA in coronal plane with traditional sequential medial release with and without measured stress applied in varus and valgus at each point of measurement through the range of flexion.

Traditionally sequential medial soft tissue release is performed for balancing in total knee arthroplasty for varus knees. Its effects on kinematics have been described in extension and 90° flexion in coronal plane. This is the first study to describe its effects on kinematics throughout flexion. 12 cadaveric knees were studied using a computer navigation system to assess kinematics. Femoro-Tibial-Mechanical-Angle(FTMA) was studied in extension, 0°, 5°, 30°,45°,60°,90° and maximum flexion. Sequential medial release was performed in 7 steps, described by Luring et al(Ref). At each step FTMA was measured without and with stressing. A 10 Newton Meter moment arm was applied for varus and valgus stress. Most of the initial release steps had little effect on FTMA without force applied, especially in the initial 60° of flexion. Application of varus force demonstrated very small changes. Application of valgus force demonstrated little change in initial arc of flexion until step 5 was reached (Table 1). Our study concludes the present sequence of medial release may not be correct and should be further investigated to modify the sequence for soft tissue balancing in TKR surgery.

Purpose of the study: Rotation of the tibial implant is an important factor for the functional outcome of total knee arthroplasty (TKA). Any rotational malposition will cause eccentric loading of the plateau. Several techniques have been recommended to avoid malposition, but none has proven superior over the others in terms of reliability or reproductibility. The landmark used to establish rotation must meet two prerequisites: easy identification and reliable representation of the anatomic rotation of the proximal tibia. This study was conducted to compare seven different techniques for landmarking used for choosing the rotation of the tibial base in TKA.

Material and methods: An optoelectronic method was used to measure 50 tibia selected among a collection of 600 skeletons. A palper was used to locate 34 distinct landmarks and institute each reference system. The groups of anatomic points were reconstructed to form lines and plans depending on the comparisons to make: posterior condylar alignment (PCA), transversal alignment (TA), anterior condylar alignment (ACA), alignment of the anterior tibial tuberosity (ATT), the transmalleolar alignment (TMA), the line of the tibial crest (LTC) and a new line, the anterior distal line (ADL). The PCA was used as the reference.

Results: Intra-observer variation was determined in a preliminary study using ten consecutive measurements. The standard deviation was 0.5° with a distribution of 1.8°. Angle: mean [-:internal rotation; +external rotation], standard deviation: difference between the minimum and the maximum. TA: −5.13; 9.2; 38.03; ACA: −12.81; 6.7; 41.74; ATT: 68.72; 8.6; 58.46; TMA: −22.68; 11.6; 72.84; LTC: 67.56; 10.3; 46.11; ADL: 16.61; 13.2; 74.93.

Discussion: This study did not prove convincingly that any one of the tibial alignments was better than another; which demonstrates that use of a single reference is probably inappropriate to determine the rotational alignment of the tibial base for TKA. It was noted however that the anterior condylar line (mean external rotation 12.8°-SD< 7° relative to the PCA) could be pertinent for future research since this line is easily accessible and palpable, particularly during navigated surgery.

As well as improved component alignment, recent publications have shown that navigation systems can assess knee kinematics and provide a quantitative measurement of soft tissue characteristics. In particular, navigation-based measures of varus and valgus stress angles have been used to define of the extent of soft-tissue release required at the time of the placement of the prosthesis. However, the extent to which such navigation-derived stress angles reflect the restraining properties of the collateral ligaments of the knee remain unknown. The aim of this cadaveric study was to investigate correlations between the structural properties of the collateral ligaments of the knee and stress angles measured with an optically-based navigation system.

Nine fresh-frozen cadaveric knees (age 81 ± 11 years) were resected 10-cm proximal and distal to the knee joint and dissected to leave the menisci, cruciate ligaments, posterior joint capsule and collateral ligaments. The resected femoral and tibial were rigidly secured within a test system which replicated the lower limb and permitted kinematic registration of the knee using the standard workflow of a commercially available image free navigation system. Frontal plane knee alignment and varus-valgus stress angles in extension were acquired. The manual force required to produce varus-valgus stress angles during clinical testing was quantified with a dynamometer attached to the distal tibial segment. Following assessment of knee laxity, bone–ligament–bone specimens were prepared and mounted within a uniaxial materials testing machine. Following 10 preconditioning cycles specimens were extended to failure. Force and crosshead displacement were used to calculate principal structural properties of the ligaments including ultimate tensile strength and stiffness as well as the instantaneous stiffness at loads corresponding to those applied during varus-valgus stress testing. Differences in the structural properties of the collateral ligaments and the varus and valgus laxity of the knee were evaluated using paired t tests, while potential relationships were investigated with scatter plots and Pearson’s product moment correlations.

There was no significant difference in the mean varus (4.3 ± 0.6°) and valgus laxity measured (4.3 ± 2.1°) for the nine knees or the corresponding distal force application required during stress testing (9.9 ± 2.5N and 11.1 ± 4.2N, respectively). Six of the nine knees had a larger varus stress angle compared to the valgus angle. There was no significant difference in the stiffness of the medial (63 ± 15 N/mm) and lateral (57 ± 13 N/mm) collateral ligaments during failure testing. The medial ligament, however, was approximately two fold stronger than its lateral counterpart (780 ± 214N verse 376 ± 104N, p< 0.001). While the laxity measures of the knee were independent of the ultimate tensile strength and stiffness of the collateral ligaments, there was a significant correlation between the force applied during stress testing and the instantaneous stiffness of the medial (r = 0.91, p = 0.001) and lateral (r = 0.68, p = 0.04) collateral ligaments.

The findings of the current study suggest that computer-assisted measures of passive knee laxity are largely independent of the ultimate strength and stiffness of the collateral ligaments. The force applied during manual stress testing of the knee, however, was strongly correlated with the instantaneous stiffness of the collateral ligaments suggesting users may attend to the low-stress behaviour of the ligaments. Nonetheless the force applied during stress testing varied between knees, as did the resultant angular deviation. Therefore to make use of the quantified data given by navigation systems, further work to understand the relationships between applied force, resultant stress angles and clinical outcomes for knee arthroplasty is required.