Effect of cup inclination on predicted contact stress-induced volumetric wear in total hip replacement.

In order to increase the lifetime of the total hip endoprosthesis, it is necessary to understand mechanisms leading to its failure. In this work, we address volumetric wear of the artificial cup, in particular the effect of its inclination with respect to the vertical. Volumetric wear was calculated by using mathematical models for resultant hip force, contact stress and penetration of the prosthesis head into the cup. Relevance of the dependence of volumetric wear on inclination of the cup (its abduction angle ϑA) was assessed by the results of 95 hips with implanted endoprosthesis. Geometrical parameters obtained from standard antero-posterior radiographs were taken as input data. Volumetric wear decreases with increasing cup abduction angle ϑA. The correlation within the population of 95 hips was statistically significant (P = 0.006). Large cup abduction angle minimises predicted volumetric wear but may increase the risk for dislocation of the artificial head from the cup in the one-legged stance. Cup abduction angle and direction of the resultant hip force may compensate each other to achieve optimal position of the cup with respect to wear and dislocation in the one-legged stance for a particular patient.

Influence of contact hip stress on the outcome of surgical treatment of hips affected by avascular necrosis.

INTRODUCTION: Biomechanical analysis is an important tool that could improve the treatment of a diseased hip. However, it is still unclear how the biomechanical status affects the clinical outcome of a certain disease. In this work we studied the long-term effect of contact hip stress on the clinical outcome of hips that were operated on by various intertrochanteric osteotomies due to avascular necrosis of the femoral head. The hypothesis being tested is that the hips with a more favourable postoperative distribution of contact hip stress have a better clinical outcome. MATERIALS AND METHODS: The study was performed on a population of 30 hips. For each hip, we determined the peak contact hip stress before the operation and immediately after the operation by using a recently developed method based on a three-dimensional mathematical model and the data from standard anteroposterior roentgenographs of both hips and pelvis. The hips were evaluated clinically 9-26 years after the operation and divided into a successful and an unsuccessful group. The average change of the peak stress due to the operation was calculated for each group, and the values were compared by t-test. RESULTS: In the successful group the operation caused an average decrease of the peak hip stress of about 10%, while in the unsuccessful group the operation caused an average increase of the peak hip stress of about 4%, the difference between the respective changes of the peak stress due to the operation being statistically significant ( p=0.001). CONCLUSION: Our results support the hypothesis that the hips with a more favourable postoperative distribution of contact hip stress have a better clinical outcome.

Intertrochanteric osteotomy for osteonecrosis of the femoral head.

We studied 32 hips (in 27 patients) treated by intertrochanteric osteotomy for osteonecrosis of the femoral head. Average follow-up was 17 (range 9-26) years. There were 22 men and 5 women with an average age of 39 (range 25-55) years. Eleven hips were classified as Ficat stage II, 19 as stage III and two as early stage IV. Nine hips showed collapse of the femoral head within 3 years after operation and in six hips collapse occurred after 3-8 years. In 13 hips slow progression with incipient signs of arthrosis was noted 8 years after surgery. Four hips with a moderate degree of necrosis at the time of surgery showed no radiological progression 9-26 years after operation. We recommend intertrochanteric osteotomy for patients with Ficat stage II and early stage III, provided that they still have a good range of motion in the hip.

Femoral head necrosis treated with vascularized iliac crest graft.

We reviewed 24 hips with avascular necrosis of the femoral head in 24 patients treated with vascularized iliac bone grafts 12 years after operation. In 7 patients the necrosis was classified as Ficat Stage II and in 17 patients as Stage III. Eight patients showed poor results. In 6 hips with fair results, moderate progression of the necrosis was noted at 3 to 8 years postoperatively. In 5 hips showing good results, slow progression with incipient signs of arthrosis were noted 8 years after surgery. In the remaining 5 patients with excellent results, no evidence of progression was noted 9 to 14 years postoperatively. The method described is recommended for treatment in the Ficat Stage II and early Stage III, when necrosis does not yet involve the complete femoral head.