P Vendittoli, J Girard (Montréal, Canada, Lille)
Biomechanical restoration is recognised as an important part of total hip arthroplasty (THA) procedure to reduce the joint reactive force and improve stability. The main objective of this study was to compare hip biomechanical reconstruction between surface replacement arthroplasty (SRA) and THA patients. SRA aims to re-create normal hip biomechanics; however the pathoanathomy of the hip, prosthetic component design, surgical technique and other factors may have a significant impact on the range of motion restoration attained following SRA. However, there is paucity of information on the effect of such factors.
Materiel and method
156 patients suffering from advanced hip joint degeneration were randomly assigned to two treatment groups: the THA or SRA group. All surgeries were performed through a posterior approach. Standardized pre and post operative antero-posterior radiographs of the pelvis were made, hip ROM were measured and clinical scores were assessed. Also, a computerized three-dimensional hip model was created from preoperative computerized tomography images of a patient who was scheduled for a SRA. The effects of the femoral component size, translation and orientation on the range of motion were analysed.
Results
Compared to the normal contra lateral side, the femoral offset increased on average 4.9mm (range -2.8 to 11.6mm, SD 3.3) for THA and decreased an average of 3.4mm (range -7.8 to 1.96, SD 2.12) for SRA (p=0.0001). There were no differences between the both groups in terms of clinical and subjective scores. No significant differences (p>0.05) were found in the total hip arc of motion (SRA=204.2°, THA=196.5°), arc of rotation (SRA=47.7°, THA=44.3°), flexion-extension arc (SRA=118.1, THA=120.1), abduction-adduction arc (SRA=43.1°, THA=42.9°). In the computer simulation model, increasing the SRA femoral component size led to global improvement in range of motion while translation increased range of motion in one direction but reduced it in the opposite direction. Change in the SRA femoral component orientation had minimal effects on range of motion in comparison to the effect of changes in the version of the acetabular component.
Discussion
In the RCT study, surgeons were less precise reconstructing the femoral offset in the THA group compared to the SRA group. The significant decreased femoral offset in SRA, seems inherent to the preferential valgus and lateral translation positioning of the femoral component in that technique. The excellent clinical outcome reported with SRA does not suggest that restoring normal offset is as crucial for the success of SRA.
To restore hip range of motion, surgeons performing surface replacement arthroplasty should aim to reproduce the natural femoral head-neck offset. Although increasing the femoral component size may achieve this, more acetabular bone will be resected. Knowing the specific zones of impingement of each arc of movement, selective translation of the femoral component or femoral neck osteoplasty can restore femoral neck offset in more critical areas without affecting acetabular bone stock. Over deepening of the acetabulum or leaving rim osteophytes should also be avoided to prevent impingement.