Purpose: We retrospectively reviewed the pre- and postoperative radiographs of 116 patients receiving primary THA in a high volume arthroplasty centre to evaluate technical causes for limb length discrepancy. We hypothesized that limb lengthening most commonly occurs as a result of low placement of the acetabular implant.

Method: A sample of 116 primary THA’s performed between 2005 and 2007 with complete one-year postoperative clinical outcomes scores and appropriate radiographs available on PACS were identified from a prospective arthroplasty database. Pre- and one-year postoperative AP bilateral hip radiographs were reviewed, and pre- and post-operative leg length discrepancy as well as the respective acetabular and femoral contribution to any postoperative leg length discrepancy (if present) were measured.

Results: We found that 19 THA’s out of 116 (16.4%) were lengthened greater than 8 mm. Mean difference from preoperative to postoperative leg length was 13.3 mm (SD 7.6 mm). A mean of 6.3 mm (SD 6.2 mm) in lengthening was contributed by the femoral stem, and 5.3 mm (SD 6.3 mm) of lengthening was contributed by placement of the acetabular implant (p=0.738). There was a significant correlation between lengthening of the limb and femoral placement of the stem (r=0.5, p< 0.0001). Likewise, there was a strong correlation between limb lengthening and low placement of the cup (r=0.6, p< 0.0001). Of those limbs that were lengthened greater than 8 mm, Oxford Hip Score at one-year post-operative was not correlated with over-lengthening (r=0.06, p=0.551).

Conclusion: These results support our hypothesis that limb lengthening is indeed due to low placement of the acetabular implant, and equally this was attributable to error in placement of the femoral stem. We conclude that with careful preoperative planning and intraoperative identification of the tear drop, a significant reduction in clinically relevant limb lengthening can be achieved.