It is impossible to determine the effect of a single parameter in clinical or in-vitro knee research. There are also parameters which can not or hardly be determined. These disadvantages can be overcome with a model. The objective of this study was to create a dynamic FE model of a human knee joint after TKA which is applicable to a variety of research question.

The knee model consisted of a femur, tibia and patella, collateral ligaments and a PCL, combined with a CKS cruciate retaining total knee prosthesis. The patella was not resurfaced. An axialload of 150 N and a quadriceps-force of 81N was applied. The model was validated by the model prediction of joint laxities at different flexion-angles and the calculation of the knee kinematics during flexion-extension.

The predicted varus-valgus laxity at different flexion angles was in between 0 and 6.3 degrees. Laxity values decreased towards extension and towards 90 degrees of flexion. The AP test at 20, 30 and 90 degrees of flexion showed a anterior laxity of 3.1, 4.3 and 2 mm, respectively. The posterior laxity was 5.7 mm, but could only be determined at 90 degrees. The model predicted reasonable kinematics, which were identical for two consecutive flexion-extension movements.

The model predictions were well in agreement with reported values, which were measured experimentally. Differences could be well explained by ligament structures which were (still) omitted with in the model. This dynamic model, in which ligaments were actually modelled as bands, combined all major structures within the knee joint. It was well able to predict laxities and kinematics and turned out to be very stable, mathematically. With this model we will be able to address effects of prosthetic and surgical parameters on the stability and kinematics of the knee joint.

Introduction The results of THA in patients with post-traumatic arthritis are inferior to those in nontraumatic arthritis, both after cemented and noncemented THA. This increased failure rate is caused by the bone stock loss and the abnormal anatomy. We studied the outcome of acetabular bone impaction grafting and a cemented cup in 20 cases at 3 to 18 years follow-up.

Materials and Methods Between January 1984 and January 2000 in 20 consecutive patients bone impaction grafting was used for arthritis after previous acetabular fracture, grafting was done in all for bone stock loss. There were 14 men and 6 women. The average age at THA was 53.3 years (range, 35 to 75 years). Defects were classified according to the AAOS acetabular defect classification (3 type I defects, 10 type II defects, 5 type III defects and 2 type V). Metal meshes were used in 6 medial wall and in 3 peripheral wall defects. In 15 cases autograft bone chips were used, in 5 both auto- and allograft was used. Grafts were impacted using impactors and a hammer and a PE cup was cemented. At review in January 2003 no patient was lost.

Results Two patients died during follow up after 3 and 4 years respectively, deaths were not related to surgery and data are included. The mean follow up was 9.5 years (range, 3 to 18 years). The pre-operative Harris hip score was 44 (32–61). During follow up a cup revision was performed in two cases; one for a culture proven septic loosening 14.5 years after surgery and the other for aseptic loosening at 15.3 years after surgery. Both revised cups were radiologically loose at revision. At review the 16 surviving hips had an aver. Harris hip score of 93 points (range, 62 to 100 points). Fourteen hips were free of pain, one patient had slight and one had mild pain.

Radiologically, none of these 16 cups was loose. However, two of the cups migrated more than 5 mm (e.g. 6 and 8 mm) relative to the initial post-operative X-ray. At review, both patients were symptom free.

The Kaplan-Meier survival rate of the cup with endpoint revision for any reason was 100% at ten years and 80% (95% CI, 62–98%) after 15 years; with endpoint cup revision for aseptic loosening the survival was 100% both at 10 and 15 years.

Conclusion The acetabular bone impaction grafting technique with a cemented cup is a biologically attractive technique to reconstruct the bone stock loss after posttraumatic arthritis with good long-term survival, even after long follow-up.