Stephan Rothstock

Biomechanical evaluation of two intramedullary nailing techniques with different locking options in a three-part fracture proximal humerus model☆

BACKGROUND Osteosynthesis of unstable proximal humerus fractures still remains challenging. The aim of this study was to investigate two intramedullary nailing techniques with different locking options in a three-part fracture model and prove whether two new fixation concepts, introducing additional locking screw-in-screws inserted through the head of the proximal screws, and a calcar screw, provide better stability. METHODS A biomechanical testing model for three-part proximal humerus fractures including cyclic axial loading with increasing peak load and simultaneous pulling forces at the rotator cuff was used to test 12 pairs of human cadaver humeri, assigned to four groups and instrumented with either Targon PH (T1) or MultiLoc PHN in 3 different configurations (standard M1; two additional screw-in-screw M2; one additional calcar screw and two screw-in-screw M3). FINDINGS Initial range of motion in internal-external rotation and mediolateral translation was smallest in M3 (1.82°; 0.11mm), biggest in T1 (3.63°; 0.51mm) and significantly different between these two groups (p=0.02 and p=0.04, respectively). M3 showed minimum head migration along the nail and varus tilting after 5000 cycles (0.31mm; 0.20°) and 10000 cycles (1.59mm; 0.34°). M2 and M3 performed better than M1 and T1 regarding varus collapse. The highest number of cycles to failure was observed for M3 (20733) and the lowest for T1 (10083) with significant difference between these two groups (p=0.04). INTERPRETATION The configuration with two screw-in-screw and a calcar screw was superior in most aspects. The screw-in-screws were found to contribute against varus collapse. Both new fixation concepts could provide better stability in proximal humerus fractures.

Influence of interface condition and implant design on bone remodelling and failure risk for the resurfaced femoral head

Resurfacing of the femur has experienced a revival, particularly in younger and more active patients. The implant is generally cemented onto the reamed trabecular bone and theoretical remodelling for this configuration, as well as uncemented variations, has been studied with relation to component positioning for the most common designs. The purpose of this study was to investigate the influence of different interface conditions, for alternative interior implant geometries, on bone strains in comparison to the native femur, and its consequent remodelling. A cylindrical interior geometry, two conical geometries and a spherical cortex-preserving design were compared with a standard implant (ASR, DePuy International, Ltd., UK), which has a 3° cone. Cemented as well as uncemented line to line and press-fit conditions were modelled for each geometry. A patient-specific finite element model of the proximal femur was used with simulated walking loads. Strain energy density was compared between the reference and resurfaced femur, and input into a remodelling algorithm to predict density changes post-operatively. The common cemented designs (cylindrical, slightly conical) had strain shielding in the superior femoral head (>35% reduction) as well as strain concentrations (strain>5%) in the neck regions near the implant rim. The cortex-preserving (spherical) and strongly conical designs showed less strain shielding. In contrast to the cemented implants, line to line implants showed a density decrease at the centre of the femoral head, while all press-fit versions showed a density increase (>100%) relative to the native femur, which suggests that uncemented press-fit implants could limit bone resorption.

Influence of peri-implant bone quality on implant stability

INTRODUCTION Insufficient primary stability is still reported for proximal humerus fractures in elderly patients. Fixation stability could be improved by aiming locking screws at bone volumes with better properties. The aims of this study were to investigate the bone regions engaged by the locking screws of a Proximal Humeral Nail (MultiLoc PHN), and to evaluate the influence of peri-screw bone quality on bone-nail construct stability. MATERIALS AND METHODS Twelve cadaveric humeri were divided into two groups. The distal locking part of the PHN was fixed to the specimens. The nails were removed and the bones scanned using HR-pQCT. Bone properties were evaluated at the locations where the proximal locking screws would have been positioned after complete instrumentation. A three-part fracture model was used for mechanical testing of the instrumented bones, considering axial displacement and varus deformation as parameters of interest. RESULTS The secondary locking screws targeted bone volumes in the posteromedial part of the humerus with statistically significant higher quality, thus reducing varus deformation. Significant correlation was found between axial displacement and bone properties at the primary proximal screws. Significant correlation was found between the varus deformation and apparent BMD at the secondary locking screws. CONCLUSION The findings of this study confirmed that directing the proximal locking screws at bone regions with better properties can improve fixation stability.

Primary stability of uncemented femoral resurfacing implants for varying interface parameters and material formulations during walking and stair climbing

Primary stability of uncemented resurfacing prosthesis is provided by an interference fit between the undersized implant and the reamed bone. Dependent on the magnitude of interference, the implantation process causes high shear forces and large strains which can exceed the elastic limit of cancellous bone. Plastification of the bone causes reduced stiffness and could lead to bone damage and implant loosening. The purpose in this study was to determine press-fit conditions which allow implantation without excessive plastic bone deformation and sufficient primary stability to achieve bone ingrowth. In particular, the influence of interference, bone quality and friction on the micromotion during walking and stair-climbing was investigated. Therefore elastic and plastic finite element (FE) models of the proximal femur were developed. Implantation was realized by displacing the prosthesis onto the femur while monitoring the contact pressure, plastic bone deformation as well as implantation forces. Subsequently a physiologic gait and stair-climbing cycle was simulated calculating the micromotion at the bone-implant interface. Results indicate that plastic deformation starts at an interference of 30microm and the amount of plastified bone at the interface increases up to 90% at 150microm interference. This effect did not reduce the contact pressure if interference was below 80microm. The micromotion during walking was similar for the elastic and plastic FE models. A stable situation allowing bony ingrowth was achieved for both constitutive laws (elastic, plastic) for walking and stair climbing with at least 60microm press-fit, which is feasible with clinically used implantation forces of 4kN.

Does Impaction Matter in Hip Resurfacing? A Cadaveric Study

Eight pairs of fresh frozen human femora were prepared for hip resurfacing. One side of each pair was impacted gently, the other side vigorously. After implantation procedure, specimens were loaded in a material testing machine to the ultimate fracture load. Median impaction loads on the vigorously implanted side were 11,298N compared to 1374N on the gently implanted side. Failure loads in the high-impact group (median, 8873N) were significantly (P = .0078) reduced when compared with the low-impact group (median, 9237N). The study stresses that meticulous reaming of the femoral head and the pinhole is of tremendous importance. Remaining obstacles can lead to excessive loads, while attempting to enforce the correct seating of the implant. Only careful, slight tapping should be applied to ensure final seating.

Cartilage surface characterization by frictional dissipated energy during axially loaded knee flexion-An in vitro sheep model

Cartilage defects and osteoarthritis (OA) have an increasing incidence in the aging population. A wide range of treatment options are available. The introduction of each new treatment requires controlled, evidence based, histological and biomechanical studies to identify potential benefits. Especially for the biomechanical testing there is a lack of established methods which combine a physiologic testing environment of complete joints with the possibility of body-weight simulation. The current in-vitro study presents a new method for the measurement of friction properties of cartilage on cartilage in its individual joint environment including the synovial fluid. Seven sheep knee joints were cyclically flexed and extended under constant axial load with intact joint capsule using a 6° of freedom robotic system. During the cyclic motion, the flexion angle and the respective torque were recorded and the dissipated energy was calculated. Different mechanically induced cartilage defect sizes (16 mm², 50 mm², 200 mm²) were examined and compared to the intact situation at varying levels of the axial load. The introduced setup could significantly distinguish between most of the defect sizes for all load levels above 200 N. For these higher load levels, a high reproducibility was achieved (coefficient of variation between 4% and 17%). The proposed method simulates a natural environment for the analysis of cartilage on cartilage friction properties and is able to differentiate between different cartilage defect sizes. Therefore, it is considered as an innovative method for the testing of new treatment options for cartilage defects.

Influence of cooling on curing temperature distribution during cementing of modular cobalt-chromium and monoblock polyethylene acetabular cups.

Total hip replacements for older patients are usually cemented to ensure high postoperative primary stability. Curing temperatures vary with implant material and cement thickness (30°C to 70°C), whereas limits for the initiation of thermal bone damage are reported at 45°C to 55°C. Thus, optimizing surgical treatment and the implant material are possible approaches to lower the temperature. The aim of this study was to investigate the influence of water cooling on the temperature magnitude at the acetabulum cement interface during curing of a modular cobalt-chromium cup and a monoblock polyethylene acetabular cup. The curing temperature was measured for SAWBONE and human acetabuli at the cement–bone interface using thermocouples. Peak temperature for the uncooled condition reached 70°C for both cup materials but was reduced to below 50°C in the cooled condition for the cobalt-chromium cup (P = .027). Cooling is an effective method to reduce curing temperature with metal implants, thereby avoiding the risk of thermal bone damage.

PLASTIC BONE DEFORMATION DURING UNCEMENTED HIP RESURFACING: A NUMERICAL STUDY

Cemented femoral resurfacings have experienced a revival for younger and more active patients. Future developments include the possibility of an uncemented version which requires more accurate manufacturing and implantation technique but would eliminate failures related to cementing. The success of uncemented implants depends on sufficient primary stability, which is directly related to contact pressure. During the implantation process, plastic deformation of the bone can modify the contact pressure. This study investigates the influence of plastic bone deformation during implantation of uncemented femoral resurfacings on press-fit and primary stability.