A.W. Miles

Failure of femoral head fixation: a cadaveric analysis of lag screw cut-out with the gamma locking nail and AO dynamic hip screw

The most commonly reported failure mode of sliding hip screws in published literature is cut-out of the lag screw. This study investigates the resistance to failure of the femoral head, with lag screws used in two types of sliding hip screws, the gamma locking nail (Howmedica) and the dynamic hip screw (DHS) (Synthes). The investigation consisted of biomechanical tests under static loading conditions on 12 pairs of cadaveric femoral heads, to establish the failure loads due to screw cut-out for the two implant lag screws. The gamma nail appeared to reduce the tendency to cut-out in the osteoporotic bone (soft) associated with elderly patients in whom these devices are commonly used (p < 0.05). In high density bone (hard) the gamma lag screw also appeared to be stronger, because the DHS showed a tendency to bend. The larger diameter of the gamma nail lag screw resists bending and appears to reduce the risk of cut-out compared with the DHS.

Fracture and loosening of Charnley femoral stems. Comparison between first-generation and subsequent designs

We report the results of a 4- to 17-year clinical and radiological follow-up of 264 Charnley first-generation stems in comparison with those of 402 second- and subsequent-generation stems. The incidence of fracture was 4.1% in first-generation stems and 0.5% in second- and subsequent-generation stems. The incidence of stem loosening requiring or likely to require revision was 3.1% in first-generation and 11.4% in second-generation stems. We believe that the increased loosening rate in second- and subsequent-generation stems is due to their larger cross-sectional area, which produces an increase in flexural stiffness.

Mechanical characteristics of antibiotic-laden bone cement.

We studied the mechanical characteristics of cement-antibiotic combinations in vitro. Palacos R was tested without antibiotics, with gentamicin alone and with gentamicin plus vancomycin or flucloxacillin. Palacos LV was studied only with gentamicin added. CMW I was studied with gentamicin added, with gentamicin plus vancomycin, and with gentamicin plus flucloxacillin. We performed four-point bending tests on beams of cement to establish bending strength and modulus, and compared the values to ISO standards. Density was also assessed. Palacos R was the strongest of the cements (bending strength 80 MPa). Palacos formulations (apart from Palacos LV) had a higher density and bending modulus than CMW 1. Statistical comparison of various cements with plain Palacos R showed lower density in 4 of the mixtures, and lower bending strength and modulus in 6 of the mixtures. Palacos R/gentamicin plus vancomycin and CMW 1/gentamicin plus vancomycin had bending strength slightly above minimum ISO standards, suggesting that the addition of vancomycin during cementmixing may compromise the outcome in revision surgery for sepsis.

An experimental study of the failure modes of the Gamma Locking Nail and AO Dynamic Hip Screw under static loading: a cadaveric study

The sliding compression screw is widely regarded as the optimum treatment for intertrochanteric fractures of the femur, allowing bone fragments to impact until a bony support has been established across the fracture site. This study carried out biomechanical, cadaveric tests to establish the influence of direct static loading situations on the modes of failure of the Gamma Nail compared with the Dynamic Hip Screw (DHS). Clinical studies report DHS failures of lag screws cutting-out, bending of the lag screws and cortical screws pulling out causing plate loosening. Gamma Nail failures include lag screw cut-out or fractures of the femoral shaft around the distal locking screws or nail tip. In this study each failure mode has been isolated, to establish the loads to failure under various fracture configurations. The biomechanical results indicated that the intramedullary Gamma Locking Nail can be recommended over a standard DHS in cases of subtrochanteric fracture or conditions of very poor bone quality.

Ultrasonic propagation in cortical bone mimics

Understanding the velocity and attenuation characteristics of ultrasonic waves in cortical bone and bone mimics is important for studies of osteoporosis and fractures. Three complementary approaches have been used to help understand the ultrasound propagation in cortical bone and bone mimics immersed in water, which is used to simulate the surrounding tissue in vivo. The approaches used were Lamb wave propagation analysis, experimental measurement and two-dimensional (2D) finite difference modelling. First, the water loading effects on the free plate Lamb modes in acrylic and human cortical bone plates were examined. This theoretical study revealed that both the S0 and S1 mode velocity curves are significantly changed in acrylic: mode jumping occurs between the S0 and S1 dispersion curves. However, in human cortical bone plates, only the S1 mode curve is significantly altered by water loading, with the S0 mode exhibiting a small deviation from the unloaded curve. The Lamb wave theory predictions for velocity and attenuation were then tested experimentally on acrylic plates using an axial transmission technique. Finally, 2D finite difference numerical simulations of the experimental measurements were performed. The predictions from Lamb wave theory do not correspond to the measured and simulated first arrival signal (FAS) velocity and attenuation results for acrylic and human cortical bone plates obtained using the axial transmission technique, except in very thin plates.

The influence of the stem-cement interface in total hip replacement—a comparison of experimental and finite element approaches:

Abstract Experimental and finite element investigations were carried out on axisymmetric models of the femoral component of a total hip replacement. In one instance, the interface between the stem and the surrounding bone cement was assumed to be rigidly bonded; in a second, it was allowed to slip. For the latter case, a friction coefficient of 0.2 was determined experimentally. The predictions of the finite element models demonstrated excellent agreement with the results from the experimental tests at all sites where comparisons were made, thus validating these models. The effect of stemcement slip was shown to reduce the maximum shear stress in the cement mantle by approximately 30 per cent.

Optimizing a hydroxyapatite/tricalcium-phosphate ceramic as a bone graft extender for impaction grafting

The mechanical properties of morsellized bone allografts and synthetic hydroxapatite/tricalcium-phosphate (HA/TCP) ceramic extender materials for the use in impaction grafting revision hip surgery were investigated using two test methods: a basic compression test and an endurance test in an in-vitro model of an impaction grafted femur. Formalin fixed ovine bone graft was identified as mechanically similar to fresh human bone and thus suitable as an experimental material for in-vitro testing. For 1 : 1 volumetric mixes of bone allograft and synthetic extender, the granular ceramics properties were varied in porosity, chemical composition, sintering temperature and particle size. Initial mechanical stability, a crucial prerequisite for clinical success in impaction grafting, was increased for all bone/extender mixes. A high porosity, tricalcium-phosphate rich ceramic of medium particle size and sintered at high temperatures was recognized as an optimized extender material for impaction grafting balancing the mechanical and biological demands. Using the extender without bone graft as a pure replacement is not recommended.© 2001 Kluwer Academic Publishers

Modelling the effects of different fracture geometries and healing stages on ultrasound signal loss across a long bone fracture

The effect on the signal amplitude of ultrasonic waves propagating along cortical bone plates was modelled using a 2D Finite Difference code. Different healing stages, represented by modified fracture geometries were introduced to the plate model. A simple transverse and oblique fracture filled with water was introduced to simulate the inflammatory stage. Subsequently, a symmetric external callus surrounding a transverse fracture was modelled to represent an advanced stage of healing. In comparison to the baseline (intact plate) data, a large net loss in signal amplitude was produced for the simple transverse and oblique cases. Changing the geometry to an external callus with different mechanical properties caused the net loss in signal amplitude to reduce significantly. This relative change in signal amplitude as the geometry and mechanical properties of the fracture site change could potentially be used to monitor the healing process.

The effect of post-curing chemical changes on the mechanical properties of acrylic bone cement

Total joint replacement is a procedure which gives pain relief and renewed mobility to over 50 000 people each year in the UK alone. While offering new hope to many of these people, approximately 10% of these prostheses fail within 10 years. It is thought that cement fracture could be one cause of the failure of the implant. This study was primarily concerned with the effect of storage environment and time period on the work of fracture of Simplex P bone cement. It was found that the storage conditions had a significant influence on the work of fracture of bone cement. In particular, storage at body temperatures embrittled the cement, while storage in fluid media had a plasticizing effect. These trends were related to post-curing chemical changes within the cement mass, specifically the absorption of low molecular mass species from the storage environments, and the leaching of residual monomer from the cement.

Fabrication of Porous Calcium Phosphate Bioceramics as Synthetic Cortical Bone Graft

The aim of this study was to fabricate porous Hydroxyapatite/Tricalcium phosphate (HA/TCP) bioceramics with an adequate degree of interconnected porosity combined with optimal mechanical properties. Porous HA/TCP bioceramics with interconnected porosity and the controlled pore sizes necessary to simulate natural bone tissue morphology were fabricated by a novel technique of vacuum impregnation of reticulated polymeric foams with ceramic slip. By varying the characteristics of the slips and using foams of different pores per inch (ppi), samples of porous HA/TCP, blocks and granules, with a wide range of pore sizes were successfully manufactured. Functionally gradient materials (FGM) with porosity gradients were also made and no weakness was found at the interface. The pore size of the HA/TCP bioceramics was in the range of 197 – 254 µm (for 20 ppi foam), 143 – 182 µm (for 30 ppi foam) and 105 – 127 µm (for 45 ppi foam). The compressive strengths and the apparent densities of the HA/TCP samples were in the range of 30 –170 MPa and 2.34 – 2.76 g/cm3 respectively. These results indicate that it is possible to manufacture open pore HA/TCP bioceramics with compressive strengths comparable to human bone which could be of clinical interest.