Objectives

Acetabular component orientation in total hip arthroplasty (THA) influences results. Intra-operatively, the natural arthritic acetabulum is often used as a reference to position the acetabular component. Detailed information regarding its orientation is therefore essential. The aim of this study was to identify the acetabular inclination and anteversion in arthritic hips.

Introduction:

The transverse acetabular ligament (TAL) antomy is not a well explored aspect of the hip joint with limited morphological description in the reviewed literature. It is often used as an anatomical landmark for orientation of the acetabular component in total hip arthroplasty (THA). There is debate as to whether it represents an appropriate guide to cup placement in THA. Present descriptions in orthopaedic literature conside it as a single plane structure to which the surgeon can align the cup. The aim of the current study was to investigate the morphology of the TAL and it was hypothesised that the current description of it being a plane would prove insufficient.

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.

Native anatomy of the arthritic hip is an important consideration in hip replacement surgery and implant design. Acetabular component orientation in total hip replacement (THR) is the single greatest factor that influences dislocation rate. Detailed knowledge regarding orientation of the native acetabulum is therefore essential. Native acetabular orientation in healthy hips is well documented but we could not find any papers detailing native acetabular orientation in the arthritic hip.

A commercially available computer navigation system (Orthopilot BBraun Aesculap, Tuttlingen, Germany) was used to assess acetabular inclination and anteversion in 65 hips with symptomatic arthritis requiring THR. Acetabular inclination in all hips was also measured on pre op anteroposterior pelvic radiographs.

Patients with DDH were excluded. All patients were Caucasian and had primary osteoarthritis, 29 males and 35 females. Average age 68(SD 8). Mean values as recorded by computer navigation were: inclination 51.4°(SD 7.1); anteversion 11.7°(SD 10.7). As recorded from radiographs mean acetabular inclination was 58.8°(SD 5.7). There was a difference between males and females. Mean navigated inclination: male 50.5°(SD 7.8); female 52.1°(SD 6.7). Mean navigated anteversion: male 8.3°(SD 8.7); female 14.39°(SD 11.6) Mean radiographic inclination: male 57.4°(SD 5.1) and female 59.8°(SD 6)

Natural acetabular orientation in arthritic hips falls out with the safe zones defined by Lewinnek. When compared with healthy hips, as described in current literature, the arthritic hip appears to have a smaller angle of inclination and anteversion, by approximately 5° and 10° respectively, in both males and females. This is useful when positioning the cup during surgery. The difference between males and females, particularly in terms of anteversion, should also be considered.

Total knee arthroplasty (TKA) is a common orthopaedic procedure. Traditionally the surgeon, based on experience, releases the medial structures in knees with varus deformity and lateral structures in knees with valgus deformity until subjectively they feel that they have achieved the intended alignment. The hypothesis for this study was that deformed knees do not routinely require releases to achieve an aligned lower limb in TKA.

A single surgeon consecutive cohort of 74 patients undergoing computer navigated TKA was examined. The mechanical axes were taken as the references for distal femoral and proximal tibial cuts. The trans-epicondylar axis was taken as the reference for frontal femoral and posterior condylar cuts. A soft tissue release was undertaken after the bony cuts had been made if the mechanical femoro-tibial (MFT) angle in extension did not come to within 2° of neutral as shown by computer readings. The post-operative alignment was recorded on the navigation system and also analysed with hip-knee-ankle (HKA) radiographs.

The range of pre-operative deformities on HKA radiographs was 15° varus to 27° valgus with a mean of 5° varus (SD 7.4°). Only two patients required a medial release. None of the patients required a lateral release. The post implant navigation value was within 2° of neutral in all cases. Post-operative HKA radiographs was available for 71 patients. The mean MFT angle from radiographs was 0.1° valgus (SD 2.1°). The range was from 6° varus to 7° valgus but only six patients (8.5%) were outside the ±3° range. The kinematic analysis also showed it to be within 2 degrees of neutral throughout the flexion making sure it is well balanced in 88% cases.

This series has shown that over 90% of patients had limbs aligned appropriately without the need for routine soft tissue releases. The use of computer assisted bone cuts leads to a low level of collateral release in TKA.