The comparison of density-elastic modulus equations for the distal ulna at multiple forearm positions: a finite element study.

The accuracy of an empirically derived density-modulus equation for bone depends upon the loading conditions and anatomic site of bone specimens used for experimentation. A recent study used FE modeling to compare the ability of three density-modulus relationships to predict strain during bending in neutral forearm rotation in the distal ulna; however, due to the inhomogeneous nature of these FE models, the performance of each equation is not necessarily consistent throughout forearm rotation. This issue is addressed in the present study, which compares the performance of these equations in pronation and supination. Strain gauge data were collected at six discreet locations of six ulna specimens loaded in bending at 40° of pronation and supination. Three FE models of each specimen were made, one for each density-modulus relation, and the strain output compared to the experimental data. The equation previously shown to be most accurate in predicting ulnar strain in neutral forearm rotation was also most accurate in pronation and supination. These results identify this one equation as the most appropriate for future FE analysis of the ulna (including adaptive remodeling, and further show that isotropic and inhomogeneous FE bone models may provide consistent results in different planes of bending.

Bone stresses before and after insertion of two commercially available distal ulnar implants using finite element analysis.

Distal ulnar arthroplasty is becoming a popular treatment option for disorders of the distal radioulnar joint; however, few studies have investigated how load transfer in the ulna is altered after insertion of an implant. The purpose of our study was to compare bone stresses before and after insertion of two commercially available cemented distal ulnar implants: an implant with a titanium stem and an implant with a cobalt chrome stem. Appropriately sized implants of both types were inserted into eight previously validated subject-specific finite element models, which were created by using information derived from computed tomography scans. The von Mises stresses were compared at eight different regions pre- and post-implantation. The bone stresses with the titanium stem were consistently closer to the pre-implantation stresses than with the cobalt chrome stem. For the loading situation and parameters investigated, results of these models show that insertion of the E-Centrix® ulnar Head may result in less stress shielding than the SBI uHead™ stem. Future studies are required to investigate other implant design parameters and loading conditions that may affect the predicted amount of stress shielding.

The effect of the density-modulus relationship selected to apply material properties in a finite element model of long bone.

Material property assignment is a critical step in developing subject-specific finite element models of bone. Inhomogeneous material properties are often applied using an equation relating density and elastic modulus, with the density information coming from CT scans of the bone. Very few previous studies have investigated which density-elastic modulus relationships from the literature are most suitable for application in long bone. No such studies have been completed for the ulna. The purpose of this study was to investigate six such density-modulus relationships and compare the results to experimental strains from eight cadaveric ulnae. Subject-specific finite element models were developed for each bone using micro-CT scans. Six density-modulus equations were trialed in each bone, resulting in a total of 48 models. Data from a previously completed experimental study in which each bone was instrumented with twelve strain gauges were used for comparison. Although the relationship that best matched experimental strains was somewhat specimen and location dependent, there were two relations which consistently matched the experimental strains most closely. One of these under-estimated and one over-estimated the experimental strain values, by averages of 15% and 31%, respectively. The results of this study suggest that the ideal relationship for the ulna may lie somewhere in between these two relations.

Role of an anterior flange on cortical strains through the distal humerus after total elbow arthroplasty with a latitude implant.

PURPOSE: Anterior flanges have been added to the humeral components of some total elbow arthroplasty systems to improve load transfer to the humerus and thereby reduce stress shielding in an effort to decrease the incidence of loosening. Either a wedge of bone or bone cement is placed between the anterior surface of the humerus and the flange. The purpose of this study was to quantify the cortical strains in the humerus as a function of these implantation options. METHODS: Five cadaveric distal humeri were fitted with bending strain gauges at 2 levels on the diaphysis and axial strain gauges at 1 level. Each specimen was subjected to cantilevered bending and axial compressive loads. Subsequently, a humeral prosthesis was inserted, and testing was repeated with 3 materials behind the flange: no graft (simulating an implant with no flange), a wafer of cancellous bone, and a block of bone cement. RESULTS: The presence of an anterior flange had no significant effect on load transfer through the distal humerus regardless of graft material. This was found to be consistent for both bending and axial loading modes at all gauge levels; however, the supporting collar effect of the implant may have influenced axial compression results. CONCLUSIONS: These results suggest that for the Latitude humeral component studied, the placement of bone or bone cement behind the anterior flange may not influence the cortical strains in the distal humerus under bending loads. However, a flange may still influence cortical strains using another implant with different geometric and material properties than the currently studied design.

Development of a testing methodology to quantify bone load transfer patterns for multiple stemmed implants in a single bone with an application in the distal ulna.

Optimal parameters for many orthopaedic implants, such as stem length and material, are unknown. Geometry and mechanical properties of bone can vary greatly amongst cadaveric specimens, requiring a large number of specimens to test design variations. This study aimed to develop an experimental methodology to measure bone strains as a function of multiple implant stem designs in a single specimen, and evaluate its efficacy in the distal ulna. Eight fresh-frozen cadaveric ulnae were each instrumented with 12 uniaxial strain gauges on the medial and lateral surfaces of the bone. The proximal portion of each ulna was cemented in a custom-designed jig that allowed a medially directed force to be applied to the distal articular surface. An implant with a finely threaded stem was cemented into the canal by an experienced upper extremity orthopaedic surgeon. Six loads (5-30 N) were applied sequentially to the lateral surface of the prosthetic head using a materials testing machine. Testing was repeated after breaking the stem-cement bond, and after removing and reinserting the stem several times into the threaded cement mantle. Near the end of the testing period, the initial stem was reinserted and data were collected to determine if there was any change in bone properties or testing setup over time. Finally, a smooth stem was inserted for comparison to the threaded stem. Strain varied linearly with load (R(2)> or =0.99) for all testing scenarios. Bending strains were not affected by breaking the stem-cement bond (P=0.7), testing durations up to 18 h (P=0.7), nor the presence of threads when compared to a smooth stem (P>0.4). Furthermore, for all gauges, there was no interaction between the effect of the threads and level of applied load (P>0.1). This methodology should prove to be useful to compare stem designs of varying lengths and materials in the same bone, allowing for a direct comparison between implant designs for the ulna and other bones subjected primarily to bending loads. Furthermore, it will minimize the need for large numbers of specimens to test multiple implant designs. The ultimate goal of using this protocol is to optimize implant stem properties, such as length and material, with respect to load transfer.

The effect of distal ulnar implant stem material and length on bone strains.

PURPOSE: Implant design parameters can greatly affect load transfer from the implant stem to the bone. We have investigated the effect of length or material of distal ulnar implant stems on the surrounding bone strains. METHODS: Eight cadaveric ulnas were instrumented with 12 strain gauges and secured in a customized jig. Strain data were collected while loads (5-30 N) were applied to the medial surface of the native ulnar head. The native ulnar head was removed, and a stainless steel implant with an 8-cm-long finely threaded stem was cemented into the canal. After the cement had cured, the 8-cm stem was removed, leaving a threaded cement mantle in the canal that could accept shorter threaded stems of interest. The loading protocol was then repeated for stainless steel stems that were 7, 5, and 3 cm in length, as well as for a 5-cm-long titanium alloy (TiAl(6)V(4)) stem. Other stainless steel stem lengths between 3 and 7 cm were tested at intervals of 0.5 cm, with only a 20 N load applied. RESULTS: No stem length tested matched the native strains at all gauge locations. No significant differences were found between any stem length and the native bone at the 5th and 6th strain gauge positions. Strains were consistently closer to the native bone strains with the titanium stem than the stainless steel stem for each gauge pair that was positioned on the bone overlying the stem. The 3-cm stem results were closer to the native strains than the 7-cm stem for all loads at gauges locations that were on top of the stem. CONCLUSIONS: The results from this study suggest that the optimal stem characteristics for distal ulnar implants from a load transfer point of view are possessed by shorter (approximately 3 to 4 cm) titanium stems.