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

We dissected 20 cadaver hips in order to investigate the anatomy and excursion of the trochanteric muscles in relation to the posterior approach for total hip replacement. String models of each muscle were created and their excursion measured while the femur was moved between its anatomical position and the dislocated position. The position of the hip was determined by computer navigation.

In contrast to previous studies which showed a separate insertion of piriformis and obturator internus, our findings indicated that piriformis inserted onto the superior and anterior margins of the greater trochanter through a conjoint tendon with obturator internus, and had connections to gluteus medius posteriorly. Division of these connections allowed lateral mobilisation of gluteus medius with minimal retraction. Analysis of the excursion of these muscles revealed that positioning the thigh for preparation of the femur through this approach elongated piriformis to a maximum of 182%, obturator internus to 185% and obturator externus to 220% of their resting lengths, which are above the thresholds for rupture of these muscles.

Our findings suggested that gluteus medius may be protected from overstretching by release of its connection with the conjoint tendon. In addition, failure to detach piriformis or the obturators during a posterior approach for total hip replacement could potentially produce damage to these muscles because of over-stretching, obturator externus being the most vulnerable.