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

Structural analysis of hollow versus solid-stemmed shoulder implants of proximal humeri with different bone qualities

Stress shielding of the proximal humerus following total shoulder arthroplasty (TSA) can promote unfavorable bone remodeling, especially for osteoporotic patients. The objective of this finite element (FE) study was to determine if a hollow, rather than solid, titanium stem can mitigate this effect for healthy, osteopenic, and osteoporotic bone. Using a population-based model of the humerus, representative average healthy, osteopenic, and osteoporotic humerus FE models were created. For each model, changes in bone and implant stresses following TSA were evaluated for different loading scenarios and compared between solid versus hollow-stemmed implants. For cortical bone, using an implant decreased von Mises stress with respect to intact values up to 34.4%, with a more pronounced effect at more proximal slices. In the most proximal slice, based on changes in strain energy density, hollow-stemmed implants outperformed solid-stemmed ones through reducing cortical bone volume with resorption potential by 11.7% ± 2.1% (p = .01). For cortical bone in this slice, the percentage of bone with resorption potential for the osteoporotic bone was greater than the healthy bone by 8.0% ± 1.4% using the hollow-stemmed implant (p = .04). These results suggest a small improvement in bone-implant mechanics using hollow-stemmed humeral implants and indicate osteoporosis could exacerbate stress shielding to some extent. The hollow stems maintained adequate strength and using even thinner walls may further reduce stress shielding. After further developing these models, future studies could yield optimized implant designs tuned for varying bone qualities.

The effect of static muscle forces on the fracture strength of the intact distal radius in vitro in response to simulated forward fall impacts

The distal radius fracture (DRF) is a particularly dominant injury of the wrist, commonly resulting from a forward fall on an outstretched hand. In an attempt to reduce the prevalence, costs, and potential long-term pain/deformities associated with this injury, in vivo and in vitro investigations have sought to classify the kinematics and kinetics of DRFs. In vivo forward fall work has identified a preparatory muscle contraction that occurs in the upper extremity prior to peak impact force. The present investigation constitutes the first attempt to systematically determine the effect of static muscle forces on the fracture threshold of the distal radius in vitro. Paired human cadaveric forearm specimens were divided into two groups, one that had no muscle forces applied (i.e., right arms) and the other that had muscle forces applied to ECU, ECRL, FCU and FCR (i.e., left arms), with magnitudes based on peak muscle forces and in vivo lower bound forward fall activation patterns. The specimens were secured in a custom-built pneumatic impact loading device and subjected to incremental impacts at pre-fracture (25 J) and fracture (150 J) levels. Similar fracture forces (6565 (866)N and 8665 (5133)N), impulses (47 (6)Ns and 57 (30)Ns), and energies (152 (38)J and 144 (45)J) were observed for both groups of specimens (p>0.05). Accordingly, it is suggested that, at the magnitudes presently simulated, muscle forces have little effect on the way the distal radius responds to forward fall initiated impact loading.

Prosthetic arthroplasty of the distal radioulnar joint: historical perspective and 24-year follow-up

This is a report of the first prosthetic hemiarthroplasty and full arthroplasty, designed and implanted for the distal radioulnar joint in 1988. Two case reports are presented, with follow-up of 24 years. Experience and problems in the design of both a hemiarthroplasty and total prosthetic arthroplasty are described, in the hope that future developments may avoid past failures.

Effect of stem length on prosthetic radial head micromotion

BACKGROUND

Osteointegration of press-fit radial head implants is achieved by limiting micromotion between the stem and bone. Aspects of stem design that contribute to the enhancement of initial stability (ie, stem diameter and surface coating) have been investigated. The importance of total prosthesis length and level of the neck cut has not been examined.

METHODS

Cadaveric radii were implanted with cementless, porous-coated radial head stems. We resected 10, 12, 15, 20, and 25 mm of radial neck in each specimen. Stem-bone micromotion was measured after each cut. Values were expressed in terms of quotients (cantilever quotient).

RESULTS

A threshold effect was observed at 15 mm of neck resection (cantilever quotient, 0.4), with a significant increase in micromotion observed between 12 mm (40 ± 10 μm) and 15 mm (80 ± 25 μm). A cantilever quotient of 0.35 or less predicted implant stability, whereas implants with a cantilever quotient of 0.6 or more were unstable. In between, the stems were "at risk" of instability.

CONCLUSION

Initial stem stability of a porous-coated, cementless radial head implant is dependent on length of the implant stem within bone and the level of the cut (amount of bone resected). Stability may be compromised by an implant with a combined head and neck length that is too long compared with the stem length within the canal. We found a critical ratio of exposed prosthesis to total implant length (cantilever quotient of 0.4), which puts the prosthesis at risk of inadequate initial stability. These data carry important implications for implant design and use.

Predicting Distal Radius Bone Strains and Injury in Response to Impacts Using Multi-Axial Accelerometers

Measuring a bone’s response to impact has traditionally been done using strain gauges that are attached directly to the bone. Accelerometers have also been used for this purpose because they are reusable, inexpensive and can be attached easily. However, little data are available relating measured accelerations to bone injury, or to judge if accelerometers are reasonable surrogates for strain gauges in terms of their capacity to predict bone injuries. Impacts were applied with a custom designed pneumatic impact system to eight fresh-frozen human cadaveric radius specimens. Impacts were repeatedly applied with increasing energy until ultimate failure occurred. Three multiaxial strain gauge rosettes were glued to the bone (two distally and one proximally). Two multiaxial accelerometers were attached to the distal dorsal and proximal volar aspects of the radius. Overall, peak minimum and maximum principal strains were calculated from the strain-time curves from each gauge. Peak accelerations and acceleration rates were measured parallel (axial) and perpendicular (off-axis) to the long axis of the radius. Logistic generalized estimating equations were used to create strain and acceleration-based injury prediction models. To develop strain prediction models based on the acceleration variables, Linear generalized estimating equations were employed. The logistic models were assessed according to the quasi-likelihood under independence model criterion (QIC), while the linear models were assessed by the QIC and the marginal R2. Peak axial and off-axis accelerations increased significantly (with increasing impact energy) across all impact trials. The best injury prediction model (QIC = 9.42) included distal resultant acceleration (p < 0.001) and donor body mass index (BMI) (p < 0.001). Compressive and tensile strains were best predicted by separate uni-variate models, including peak distal axial acceleration (R2 = 0.79) and peak off-axis acceleration (R2 = 0.79), respectively. Accelerometers appear to be a valid surrogate to strain gauges for measuring the general response of the bone to impact and predicting the probability of bone injury.

Failure characteristics of the isolated distal radius in response to dynamic impact loading

We examined the mechanical response of the distal radius pre-fracture and at fracture under dynamic impact loads. The distal third of eight human cadaveric radii were potted and placed in a custom designed pneumatic impact system. The distal intra-articular surface of the radius rested against a model scaphoid and lunate, simulating 45° of wrist extension. The scaphoid and lunate were attached to a load cell that in turn was attached to an impact plate. Impulsive impacts were applied at increasing energy levels, in 10 J increments, until fracture occurred. Three 45° stacked strain gauge rosettes were affixed along the length of the radius quantifying the bone strains. The mean (SD) fracture energy was 45.5 (16) J. The mean (SD) resultant impact reaction force (IRFr) at failure was 2,142 (1,229) N, resulting in high compressive strains at the distal (2,718 (1,698) µε) and proximal radius (3,664 (1,890) µε). We successfully reproduced consistent fracture patterns in response to dynamic loads. The fracture energy and forces reported here are lower and the strains are higher than those previously reported and can likely be attributed to the controlled, incremental, dynamic nature of the applied loads.

EFFECTS OF GEOMETRY OF THE INTRAMEDULLARY STEM OF THE ULNA COMPONENT OF HINGED ELBOW JOINT PROSTHESES ON THE BONE AND IMPLANT BENDING STRESSES

Cemented intramedullary stems are commonly used in total elbow arthroplasty and a considerable amount of time and money are spent for the design of such devices. In the endeavor of reducing production cost of implants, it is of particular interest as to how the geometry of the intramedullary stem can be altered in order to reduce the amount of raw material used in the manufacture of these components. The main aim of this study was to establish a simple mechanics model in preliminary designing of an elbow joint, so that the effects of the geometry of the intramedullary stem of the ulna component of hinged elbow joint prostheses on the bending stress distribution in the ulna bone and the prosthesis stem during static loading after cemented fixation of the implant can be readily estimated. Two mathematical models, namely (i) linear load transfer model and (ii) beams on elastic foundation model were used. The material considered is biocompatible stainless steel (316L). The locations of maximum bending stress occurrence were identified. Special attention was given toward identifying the locations of stress concentrations as well as the degree of stress shielding expected with various stem geometries. The results were compared to that obtained using well-established finite element analysis techniques and the beams on elastic foundation model were chosen to interpret the stresses. It was found that reducing the length of the intramedullary stem or alternatively tapering the distal end of the stem results in a considerable reduction of stress shielding and renders the bending stress distribution in the bone to be more natural. The use of a tapered stem of rectangular cross section showed a 50% reduction in material usage and a reduction of stress shielding compared to that for a stem of uniform circular cross section. Tapered rectangular sections gave the best results in terms of functionality and cost effectiveness. This observation agrees very well with the rationale of implant design that is practiced over the years. The stresses found using this study can be used for preliminary checks against yield and fracture of the stem material and bone material.

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.

Stress distribution on the screws in posterior lumbar fusion of isthmic spondylolisthesis with 2- or 3-vertebra fixation techniques: a biomechanical cadaveric study

BACKGROUND

Two- or three-vertebra fixation techniques are both used in the treatment of spondylolisthesis. However, the number of spinal segments that should be implanted in spondylolisthesis reduction and fixation is still controversial, and there are no published reports on stress distribution on the screws with 2- or 3-vertebra fixation techniques. Understanding stress distribution in screws would be of potential great clinical importance and supply more biomechanical evidence in surgery. The aim of this study was to compare and quantitatively analyze the stress distribution on the screws in 2- or 3-vertebra fixation techniques in cadaveric models of spondylolisthesis.

MATERIALS AND METHODS

Sixteen fresh specimens of human lumbar spines were used in this study. The spondylolisthesis model was generated by Panjabi method and fixed with the SINO universal spine system by 2- (group A) or 3-vertebra (group B) fixation technique. Rectangular electrical resistance strain gauges were fixed at upper and lower surface of the root of screws bilaterally. The samples were tested under flexion/extension, left/right lateral bending, and axial compression loading. Stress on the screws was measured by strain gauge monitor, respectively.

RESULTS

Under the five different loading conditions, the stress could be compressive stress or tensile stress. Under the compression, flexion, and bending loading condition, the stress in reduction screws in group A is higher than in group B (P < 0.01). However, under the extension loading condition, stress of lower surface in reduction screws in group A is 49% lower than in group B. With regard to the anchor screws, under flexion and lateral bending conditions, stress in group A is lower than in group B (P < 0.05). Under compression and extension loading conditions, stress in group A is slightly higher than in group B, but no significant difference is detected.

CONCLUSIONS

In most loading conditions, stress in reduction screws in 2-vertebra fixation technique was higher than in 3-vertebra fixation technique. The 3-vertebra fixation technique might effectively reduce stress on the reduction screws, and decrease the probability of fatigue fractures of the screws.

Development of a customized density—modulus relationship for use in subject-specific finite element models of the ulna

Assigning an appropriate density—modulus relationship is an important factor when applying inhomogeneous material properties to finite element models of bone. The purpose of this study was to develop a customized density—modulus equation for the distal ulna, using beam theory combined with experimental results. Five custom equations of the form E = aρb were used to apply material properties to models of eight ulnae. All equations passed through a point (1.85, Ec), where ρ = 1.85 g/cm3 represents the average density of cortical bone. For custom equations (1) to (3), Ec was predicted using beam theory, and the value of b was varied within the range reported in the literature. Custom equations (4) and (5) used other values of Ec from the literature, while keeping b constant. Results obtained from the custom equations were compared with those from other equations in the literature, and with experimental results. The beam theory analysis predicted Ec = 21 ± 1.6 GPa, and the three custom equations using this value tended to have the lowest errors. The power of the equations did not affect the results as much as the value used for Ec. Overall, a customized density—modulus relationship for the ulna was generated, which provided improved results over using previously reported density—modulus equations.