Abstract
Introduction
Malalignment of cup in total hip replacement (THR) increases rates of dislocation, impingement, acetabular migration, pelvic osteolysis, leg length discrepancy and polyethylene wear. Many surgeons orientate the cup in the same anteversion and inclination as the inherent anatomy of the acetabulum. The transverse acetabular ligament (TAL) and acetabular rim can be used as a reference. No study has yet defined the exact orientation of the TAL. The aim of this study was to describe the orientation of acetabular margin and compare it with TAL orientation.
Materials and Methods
Sixty eight hips with osteoarthritis undergoing THR with computer navigation were investigated. Anterior pelvic plane was registered using anterior superior iliac spines and pubic symphysis. Orientation of the natural acetabulum as defined by the acetabular rim with any osteophytes excised was measured. Since TAL is a rectangular band like structure, three recordings were done for each corresponding to the outer middle and inner margin of the band. All the readings were given by software as radiological anteversion and inclination.
Results
All patients were Caucasian, 30 males and 38 females with mean age 67.4 years (SD 9.6) and BMI 30 (SD 5). Inclination was 54.7(SD7.9), 53(SD6.9), 47.5(SD6.8), 42.1(6.7) and anteversion 5.7(SD8.7), 5.4(SD9.9), 9.7(SD9.6), 13.5(SD9.4) for acetabular rim, outer, middle and inner borders of the TAL respectively.
For inclination TAL outer border was not significantly different to acetabular rim (mean difference 1.7°, 95%CIs −0.2° to 3.6°, p=0.082) but the middle (mean difference 7.3°, 95%CIs 5.6° to 8.9°) and inner (mean difference 12.6°, 95%CIs 11.0° to 14.2°) borders were (both p<0.001).
For anteversion TAL outer border was not significantly different to acetabular rim (mean difference 0.2°, 95%CIs −1.3° to 1.8°, p=0.758) but the middle and inner borders were (mean difference −4.0° 95%CIs −5.5° to −2.5° and −7.9°, 95%CIs −9.6° to −6.1° respectively, both p<0.001).
Anteversion for males was significantly lower than females with a mean difference of 4 for the rim and 5.7, 4.8 and 5.1 for the TAL outer, middle and inner margins respectively. Overall 57,53,40&26 of 68 patients had a combined inclination and anteversion of the native acetabulum that fell outside the “safe zone” of Lewinnek with acetabular rim, outer, middle and inner margins of TAL respectively. Compared to Lewinnek safe zones for inclination TAL inner margin performed best with 14.7% outliers and acetabular rim performed worst with 72% outliers. For anteversion TAL inner margin performed best with 25% outliers while outer margin of TAL performed worst with 39.7% outliers.
Conclusion
Orientation of the acetabulum differs a lot between individuals. The TAL middle and inner margins differ in orientation as compared to acetabular rim and TAL outer border. TAL inner border provides the best bet for placing the cup in Lewinnek's safe zone.
When using the natural acetabular orientation or TAL as a guide, it should not be assumed this will orientate the cup in Lewinnek safe zone although the validity of safe zones itself is questionable. Variation between patients must be taken into account and the difference between males and females, particularly in terms of anteversion, should be considered.
