The sagittal stem alignment and the stem version clearly influence the impingement-free range of motion in total hip arthroplasty: a computer model-based analysis

Purpose

The component alignment in total hip arthroplasty influences the impingement-free range of motion (ROM). While substantiated data is available for the cup positioning, little is known about the stem alignment. Especially stem rotation and the sagittal alignment influence the position of the cone in relation to the edge of the socket and thus the impingement-free functioning. Hence, the question arises as to what influence do these parameters have on the impingement-free ROM?

Methods

With the help of a computer model the influence of the sagittal stem alignment and rotation on the impingement-free ROM were investigated. The computer model was based on the CT dataset of a patient with a non-cemented THA. In the model the stem version was set at 10°/0°/−10° and the sagittal alignment at 5°/0°/−5°, which resulted in nine alternative stem positions. For each position, the maximum impingement-free ROM was investigated.

Results

Both stem version and sagittal stem alignment have a relevant influence on the impingement-free ROM. In particular, flexion and extension as well as internal and external rotation capability present evident differences.

In the position intervals of 10° sagittal stem alignment and 20° stem version a difference was found of about 80° in the flexion and 50° in the extension capability. Likewise, differences were evidenced of up to 72° in the internal and up to 36° in the external rotation.

Conclusions

The sagittal stem alignment and the stem torsion have a relevant influence on the impingement-free ROM. To clarify the causes of an impingement or accompanying problems, both parameters should be examined and, if possible, a combined assessment of these factors should be made.