Evaluation of Bernese periacetabular osteotomy: prospective studies examining projected load-bearing area, bone density, cartilage thickness and migration.

The typical dysplastic hip joint is characterised by maldirection of the acetabulum and femoral neck, insufficient coverage of the femoral head focally and globally and erosions of the limbus acetabuli (1). An unknown number of persons with hip dysplasia will suffer from pain in hip or groin, decreased hip function and development of osteoarthritis at a young age. The Bernese periacetabular osteotomy is performed to prevent osteoarthritis in patients with hip dysplasia and has been carried out at Aarhus University Hospital, Denmark since 1996 with more than 500 osteotomies performed. Throughout the years, research and quality improvement of the treatment has taken place and this PhD thesis is part of that process. The aims of this PhD thesis were to evaluate outcome aspects after periacetabular osteotomy in terms of I) estimating the projected loadbearing surface before and after periacetabular osteotomy, II) estimating bone density changes in the acetabulum after periacetabular osteotomy, III) developing a technique to precisely and efficiently estimate the thickness of the articular cartilage in the hip joint and IV) examining the stability of the re-orientated acetabulum after periacetabular osteotomy. In study I, we applied a stereologic method based on 3D computed tomography (CT) to estimate the projected loadbearing surface in six normal hip joints and in six dysplastic hips. The dysplastic hips were CT scanned before and after periacetabular osteotomy. We found that the average area of the projected loadbearing surface of the femoral head preoperatively was 7.4 (range 6.5-8.4) cm2 and postoperatively 11 (9.8-14.3) cm2. The area of the projected loadbearing surface was increased significantly with a mean of 49% (34-70%) postoperatively and thus comparable with the load-bearing surface in the normal control group. Double measurements were performed and the error variance of the mean was estimated to be 1.6%. The effect of overprojection, on the projected loadbearing surface was minimal. Consequently, the stereo-logic method proved to be precise and unbiased. The study indicates that this method is applicable in monitoring the loadbearing area in the hip joint of patients undergoing periacetabular osteotomy. In study II, a method based on CT and 3D design-based sampling principles was used to estimate bone density in different regions of the acetabulum. Baseline density was measured within the first seven days following periacetabular osteotomy and compared with density two years postoperatively. Double measurements were performed on three patients, and the error variance was estimated to be 0.05. Six patients with hip dysplasia scheduled for periacetabular osteotomy were consecutively included in the study. Bone density increased significantly in the anteromedial quadrant of the acetabulum as well as in the posteromedial quadrant between the two time-points. In the anterolateral quadrant bone density was unchanged following surgery, and the same was true for the posterolateral quadrant. We suggest that the observed increase in bone density medially represents a remodelling response to an altered load distribution after periacetabular osteotomy. The described method is a precise tool to estimate bone density changes in the acetabulum. Study III. As periacetabular osteotomy is performed on dysplastic hips to prevent osteoarthritic progression, changes in the thickness of the articular cartilage is a central variable to follow over time. 26 dysplastic hips on 22 females and 4 males were magnetic resonance imaged (MRI) preoperatively. The first 13 patients were examined twice, with complete repositioning of the patient and set-up in order to obtain an estimate of the precision of the method used. To show the acetabular and femoral cartilages separately, an ankle traction device was used during MRI. This device pulled the leg distally with a load of 10 kg. The mean thickness of the acetabular cartilage was 1.26 mm, SD 0.04 mm. The mean thickness of the femoral cartilage was 1.18 mm, SD 0.06. The precision calculated as the error variance was estimated for the thickness of the acetabular cartilage to 0.01 and femoral cartilage 0.02. We suggest that the method can be advantageous for assessing the progression of osteoarthritis in dysplastic hips after periacetabular osteotomy. In study IV, 32 dysplastic hips, 27 females and 5 males were included in the study. Radiostereometric examinations (RSA) were done at one week, four weeks, eight weeks and six months. Data are presented as mean + SD. Six months postoperatively, the acetabular fragment had migrated 0.7 mm + 0.8 medially, and 0.7 mm + 0.5 proximally. Mean rotation in adduction was 0.5 degrees + 1.3. In other directions, mean migration was below 0.5 mm/degrees. There was no statistical difference between migration 8 weeks and 24 weeks postoperatively in translation or rotation. Due to the limited migration, we find our postoperative partial weight-bearing regime safe. In conclusion, the studies in the present PhD thesis indicate that the projected loadbearing area of the hip joint increases considerable in patients undergoing periacetabular osteotomy and a method to estimate this area was described. Bone density increases in the medial quadrants two years postoperative and a method is developed to precisely estimate bone density on CT images. Also a method to precisely estimate cartilage thickness was presented and we suggest that the method can be advantageous for assessing the progression of osteoarthritis in dysplastic hips after periacetabular osteotomy. Due to the very limited migration of the acetabular fragment fixated with two screws, we find our fixation sufficient and the postoperative partial weight-bearing regimen safe.