Abstract
INTRODUCTION:
The 3D shape of the normal proximal femur is poorly described in current designs of proximal femur prosthesis. Research has shown that in current implant designs with small diameter femoral heads the moment arm of the ilio-psoas tendon is reduced causing weakness in full extension, while large femoral heads cause psoas tendon impingement on the femoral head neck junction [1]. The femoral head-neck junction thus directly influences the hip flexor muscles' moment arm. Mathematical modeling of proximal femoral geometry allowed a novel proximal femur prosthesis to be developed that takes into account native anatomical parameters. We hypothesized that it is possible to fit a quadratic surface (e.g. sphere, cylinder…) or combinations of them on different bone surfaces with a relatively good fit.
METHODS:
Forty six ‘normal’ hips with no known hip pathology were segmented from CT data. Previous research has shown the femoral head to have a spherical shape [2], the focus here was therefore mainly on the neck. The custom-written minimization algorithm, using least squares approximation methods, was used to optimize the position and characteristics of the quadratic surface so that the sum of distances between a set of points on the femoral neck and the quadratic surface was minimized. Furthermore, to improve upon current design regarding the transition between head and the neck, we recorded the position of the head neck articular margin in addition the slope of the transition from head to neck in the above 46 hips.
RESULTS:
The femoral neck was found to be represented with a good fit as a quadratic surface (hyperboloid) with an average root mean square error of 1.0 ± 0.13 among 46 hips. The femoral head was spherical with a mean ratio of 22.6 ± 1.75 mm. The shape of the femoral articular margin is a reproducible sinusoidal wave form, which appears to have two facets, one anterior and the other posterior. A sigmoid curve, provided by the Logistic Function was used to switch smoothly from the spherical head function to the hyperboloid neck function (Fig. 1). This curve provides a continuous mathematical function to describe the head/neck geometry.
DISCUSSION:
Traditional designs that liken the femoral head to a sphere are an oversimplification of normal hip morphology. The precise shape of the neck and the relationship of the neck to the head are the basis of this invention. The prosthesis is designed to avoid soft tissue impingement and can be optimised in shape and size to match the patient's native morphology. Neck diameter and length can be designed to achieve the optimum head-neck ratio to further improve the range of motion produced. With the current design the pain observed due to ilio-psoas impingement to implant will be reduced. Furthermore as the implant is anatomical the function of muscles and their moment arm will be unaffected.
