MORPHOLOGICAL AND BIOMECHANICAL STUDIES IN ASIAN HIPS : A CADAVERIC AND CLINICAL STUDY IN RELATION TO OSTEOARTHRITIS

Abstract

Hip disorders including primary osteoarthiritis are rare in Asians. The cause for the low incidence is not clear. It has been postulated that frequent squatting habits may have a role to play in increasing joint lubrication through increased joint contact. Very little work has been done on the morphology and joint contact patterns of the Asian hip. This study has looked into some aspects of the aetiopathogenesis and low incidence of osteoarthritis of the hip in Asians in Singapore. Sixty fresh elderly cadaveric hips were examined to study surface degenerative changes and presence of osteophytes in the various zones of the femoral head and acetabulum. Lesions were graded and recorded as either progressive or nonprogressive changes. Twenty fresh femoral heads obtained at surgery were studied under light microscopy and scored according to Mankin's grading system. Histological studies were carried out on the apparently normal and fibrillated areas of the articular cartilage. Ten of these hips were also studied in detail with scanning and transmission electron microscopy. The elderly Asian hips maintained remarkably good surface morphology with few osteophyte formation. Minimal age related degenerative changes of limited progression were mainly seen over the supero-posterior (SP) zone of the acetabulum. Histological studies showed a good low score on the Mankin's grading system. The ultra-structure of the hip articular cartilage was well maintained in most specimens. Hip joint contact studies were done in thirty-two elderly Asians. Contact pattern on manual loading in the neutral (erect) and simulated squatting position of the hip was studied using oil-based enamel paint. The hips were fixed to a specially designed jig and loaded on a load testing machine to study contact patterns in the neutral (erect) position at successive loads from 200 Newtons to 1800 Newtons. The contact patterns in the squatting position were mostly over the acetabular fat pad and partly over the supero-posterior zone of the acetabulum where the infero-medial zone of the femoral head, rather than the dome made contact. Joint contact on anatomical loading was usually limited to isolated areas over the supero-posterior zone of the hip which also showed surface degenerative changes microscopically. Total hip joint contact was not demonstrated, even when the specimens were loaded to 1800 Newtons and over, thus showing that normal cartilage compliance and physiological joint incongruity was maintained in the elderly Asian. A clinical and cadaveric radiological study was also done to analyse the morphological features of the Asian hip. In the clinical study, ninety adult volunteers, who had no previous hip or other joint complaints had a standard antero-posterior radiograph of the pelvis showing both hips. The congruity and sphericity of the femoral head was measured to determine the femoral head tilt ratio (FHR). Ten elderly habitual squatters with two control groups of ten elderly and ten young volunteers, who infrequently squat had radiographs of the hips in a squatting position. The radiological features of the hip in this position has previously not been described. In the cadaveric study, thirty hip joints were sectioned in the coronal plane at 5mm slices to note the trabecular pattern in the femoral head and acetabulum. The femoral head tilt ratio (FHR) in the one hundred and eighty hips analysed was well within normal limits with a mean of 1.08. In elderly habitual squatters, the ischial acetabular trabeculae were noted to be more prominent than in the elderly group and young adults who infrequently squat. No previous mention has been drawn in respect to these trabeculae in the literature. It may be hypothesised that overall distribution of the hip joint forces may be equalised in patients who frequently sit, stand and squat, thereby lowering joint overload and reducing the incidence of osteoarthritis of the hip.