Richard Eddy Field

Bone remodeling around the Cambridge cup: A DEXA study of 50 hips over 2 years

Background In a prospective 2-year study we have used dual-energy X-ray absorptiometry to measure periprosthetic bone mineral density (BMD) following implantation of a novel, “physiological”, acetabular component designed using composite materials. Method The acetabular components were implanted in hydroxyapatite (HA) and HA-removed options. They were implanted in conjunction with a cemented femoral component in 50 female patients who presented with displaced, subcapital, fractures of the neck of the femur. Regions of interest (ROI) were defined according to De Lee and Charnley. BMD during follow-up was compared with immediate postoperative values for the affected limb. Results The mean precision error (CV%) was 1.01%, 2.26% and 1.12%, for ROI I, II and III respectively. The mean change in BMD, for both cups, was analyzed. There was no significant difference between the BMD changes induced with the HA- and non-HA-coated cups. Interpretation After an initial fall in BMD in all 3 ROI at 6 months, ROI I and ROI II showed return to baseline BMD by 2 years. ROI III showed no significant decrease in BMD beyond 6 months, but did not return to baseline levels. Statistical analysis revealed no significant decrease in BMD in ROI I and ROI II at 2 years, compared with immediate postoperative values. The changes in BMD reflect a pattern of maximally reduced stress in the non-weight-bearing zone (ROI III), with preservation of bone density in weight bearing zones ROI I and ROI II. These results support the design principles of the Cambridge cup.

Stress shielding and stress concentration of contemporary epiphyseal hip prostheses

Abstract After the first early failures, proximal femoral epiphyseal replacement is becoming popular again. Prosthesis-to-bone load transfer is critical for two reasons: stress shielding is suspected of being responsible for a number of failures of early epiphyseal prostheses; stress concentration is probably responsible of the relevant number of early femoral neck fractures in resurfaced patients. The scope of this work was to experimentally investigate the load transfer of a commercial epiphyseal prosthesis (Birmingham Hip Replacement (BHR)) and an innovative prototype proximal epiphyseal replacement. To investigate bone surface strain, ten cadaveric femurs were instrumented with 15 triaxial strain gauges. In addition the cement layer of the prototype was instrumented with embedded gauges to estimate the strain in the adjacent trabecular bone. Six different loading configurations were investigated, with and without muscles. For the BHR prosthesis, significant stress shielding was observed on the posterior side of the head—neck region (the strain was halved); a pronounced stress concentration was observed on the anterior surface (up to five times in some specimens); BHR was quite sensitive to the different loading configurations. For the prototype, the largest stress shielding was observed in the neck region (lower than the BHR; alteration less than 20 per cent); some stress concentration was observed at the head region, close to the rim of the prosthesis (alteration less than 20 per cent); the different loading configurations had similar effects. Such large alterations with respect to the pre-operative conditions were found only in regions where the strain level was low. Conversely, alterations were moderate where the strain was higher. Thus, prosthesis-to-bone load transfer of both devices has been elucidated; the prototype preserved a stress distribution closer to the physiological condition.

Assessment of femoral neck fracture risk for a novel proximal epiphyseal hip prosthesis

BACKGROUND This study addresses the risk of femoral neck fracture associated with resurfacing hip prostheses. A novel cemented Proximal Epiphyseal Replacement (PER) featuring a short curved stem was investigated. METHODS Seven pairs of femurs were in vitro tested. One femur of each pair was randomly assigned for PER implantation. The contralateral femur was tested intact. All femurs were loaded to failure in a validated, physiological configuration. High-speed videos (10,000-12,000 frames/s) were acquired to identify the location of fracture initiation. For comparison, data were included from Birmingham Hip Resurfacing previously tested in an identical fashion (N=3). FINDINGS Relative to the contralateral intact femurs, the failure load of the PER and Birmingham implants was 15.4% higher and 10.0% lower, respectively. In six of the seven PER implants, fracture initiation (neck or inter-trochanteric) was similar to the contralateral intact femurs, suggesting comparable stress distribution. Conversely, fracture initiation in the Birmingham implants occurred at the lateral prosthesis rim, which differed substantially from the intact femurs. No correlation existed between bone quality and strengthening/weakening effect of the PER (failure load of implant as a percentage of intact: R^2=0.067). Conversely, Birmingham implantation weakened the femurs with lower density (R^2=0.92). Therefore, unlike most resurfacing prostheses, the PER seems suitable also for osteoporotic subjects. INTERPRETATION This study seems to confirm that resurfacing with a Birmingham Hip tends to reduce the strength of the proximal femur. The opposite seemed to happen with the PER, which slightly reduced the risk of neck fracture, also in low-quality bones.

Pre-clinical evaluation of the Cambridge acetabular cup

It is postulated that the stifness of current acetabular designs compromises long-term component stability. We present a novel acetabular component design that is horseshoe shaped and has a large diameter bearing. It is made from composite materials and is designed to match the stifness of subchondral bone. It is intended that stress shielding will be minimised and that the distribution of stress will be improved. The mechanical and biological suitability of the composite has been confirmed. A range of standard and non-standard, pre-clinical, tests have established the robustness and safety of the new component. The efficacy of the new design has been evaluated by clinical trial on 50 patients. Optimal results were obtained using the hydroxyapatite (HA) coated cups. Our results support the new design concept, with the caveat that biological fixation is imperative. Minor design modifications are recommended.

Anatomy: Capsule and Synovium

The hip joint is encapsulated by a thick, cylindrical, fibrous sleeve. It arises from the outer margin of the acetabulum and inserts into the metaphyseal region of the proximal femur. Reinforced by distinct ligaments, taught at extremes of motion, the capsule functions to constrain the hip joint (Hewitt et al., J Arthroplasty 17:82–9, 2002; Martin et al., Arthroscopy 24:188–95, 2008; Ito et al., J Orthop Res 27:989–95, 2009) and helps maintain joint congruence (Ito et al., J Orthop Res 27:989–95, 2009; Myers et al., Am J Sports Med 39:85S–91S, 2011; Crawford et al., Clin Orthop Relat Res 465:16–22, 2007). A synovial membrane lines the articular surface, and loose connective tissue surrounds the exterior, supporting its vascular supply (Gray’s anatomy, Philadelphia, 1918). Known to have mechanical, proprioceptive, and nociceptive properties, recent work suggests that the capsule also contributes to the stability of the femoral head within the acetabulum (Ito et al., J Orthop Res 27:989–95, 2009) and plays an integral role in circulation of synovial fluid within the joint (Field and Rajakulendran, J Bone Joint Surg Am 93:22–7, 2011).