Introduction

Geometric variations of the hip joint can give rise to abnormal joint loading causing increased stress on the articular cartilage, which may ultimately lead to degenerative joint disease. In-vitro simulations of total hip replacements (THRs) have been widely reported in the literature, however, investigations exploring the tribology of two contacting cartilage surfaces, and cartilage against metal surfaces using complete hip joint models are less well reported.

The aim of this study was to develop an in-vitro simulation system for investigating and comparing the tribology of complete natural hip joints and hemiarthroplasties with THR tribology. The simulation system was used to assess natural porcine hip joints and porcine hemiarthroplasty hip joints. Mean friction factor was used as the primary outcome measure to make between-group comparisons, and comparisons with previously published tribological studies.

Introduction

Contact between the femoral head and rim of the acetabular liner in total hip replacements has been linked to adverse tribological performance that may potentially shorten the lifespan of the prosthesis. Predicting the size and location of the contact area can be done computationally, however, experimental validation of these models is challenging due to the conforming nature of the bearing surfaces.

This study aimed to develop a method of accurately determining the in-vitro contact area between the femoral head and acetabular cup in metal-on-polyethylene and ceramic-on-polyethylene bearings under different component orientations.

Background Data: Radiography has been the mainstay of patient monitoring in scoliosis, but there is an increasing demand for its reduction to specific situations where treatment is to be decided or modified. There is concern that substitution of traditional methods with clinical impression and surface topography might not be feasible or safe.

Study Design: An outcome study of a year’s intake of new patients with adolescent idiopathic scoliosis using a protocol derived from experience with surface topography.

Method: Tolerance limits for observer and subject variation and observed changes over time were established and correlated with recorded Cobb angle changes. A “derived Cobb angle” was calculated from topographic spinal angles and radiographs of 75 patients with non-congenital scoliosis and tested against 141 similar patients. Cobb angle = 15.3 + 1.22* topographic spinal angle. A protocol was adopted with topography at every clinic visit, radiography reserved for cases with severe deformity, additional symptoms or where surgical intervention was considered. This protocol was tested on new adolescent idiopathic scoliosis (AIS) presentations to the general clinic in a single year (2001) with regard to status at presentation and outcome (n=49).

Results: Measurement error, on the average of four repositioned scans on 105 consecutive patients rounded up to 10 units on all parameters. In 75 patients with non-congenital scoliosis, change ≥10° in Cobb angle was always accompanied by a similar change on at least one topographic parameter. The mean difference was −3.9°, SD 14.7, and was greater in very small, larger or double curves and in obese patients. There was significant correlation (p< 0.01) between changes in the Cobb angle over time and that derived from the spinal angle. 49 girls presenting with a presumptive diagnosis of AIS were diagnosed thus: Normal, n=8, 4 after radiograph, all now discharged; Asymmetry, n=24, no radiographs, 11 discharged immediately, 10 after 0.5 – 1.5 years, 3 lost; AIS, n=17, Cobb angle 10–93°, 5 surgery, 6 discharged, 4 currently followed, 2 non-attendees.

Discussion: The incomplete correlation is acceptable, since within-subject variation of the Cobb angle is unknown but the observer variation was shown by Carman et al (JBJS 72(A):328–333) to be over 8°. The discrepancy between actual and derived Cobb angles at the extremes is understandable as small curves are inflated by the obligatory constant, while increased subcutaneous tissue smoothes the surface, and both double and large curves show more rotation of vertebral bodies than of spinous processes. This can be tolerated because in small curves, prediction is made on maturity indicators rather than Cobb angle, while at higher values, cosmesis is the issue, small changes in Cobb angle are less relevant, and pubertal status determines progression potential more effectively than radiographic measures.

Conclusion: Topography and reduced use of radiography allows safe monitoring of adolescent idiopathic scoliosis. It provides a validated cosmetic score which documents deformity progression, is an adjunct to clinical decision making and is mathematically related to the Cobb angle. Basic clinical modalities and careful consideration of every patient on an individual basis are still essential.