Finite element analysis provides models to identify crucial forces and phenomena
Terry Stanton & Madeleine Lovette
The dislocation rate in total hip replacement is sometimes as high as 5 percent for primary procedures and 10 percent for revisions. Impingement in total hip arthroplasty (THA), along with subluxation, is also an appreciable problem with large societal and financial costs. “Although instability now stands as the single most common reason for revision surgery, the biomechanical factors responsible for impingement, subluxation, and dislocation remain underinvestigated relative to their burden of morbidity,” said Thomas D. Brown, PhD, of the University of Iowa.
Existing approaches to studying the hip are not very applicable to research on impingement and dislocation. Such research needs to take place in controlled settings that are reasonably representative of clinical circumstances so that individual influence factors can be systematically studied. The group, therefore, had to develop and validate several new research methodologies. It relied heavily on advanced finite element formulations, computer models that could be stressed and analyzed for specific results.
Dr. Brown and his colleagues developed a series of computer-generated, multi-dimensional engineering models, using a variety of inputs and measurements—cadaver, patient physiologic motion and imaging, and physical experiments with implants—to study, predict, and validate implant performance. They relied heavily on these models to examine how implant design factors, surgical positioning factors, and patient motion challenges influence THA impingement and dislocation (Fig. 1).
Specific areas of focus included the following:
- Identifying the biomechanic challenges posed by dislocation-prone patient activities
- Quantifying design parameter effects and component surgical positioning effects for conventional metal-on-polyethylene implant constructs and the impingement/dislocation behavior of nonconventional constructs
- Quantifying the stabilizing role of the hip capsule (and of surgical repairs of capsule defects)
- Systematically studying impingement and edge loading of hard-on-hard bearings, fracture of ceramic liners, confounding effects of patient obesity, and subluxation-mediated worsening of third-body particle challenge
Kinetic, kinematic factors
Dr. Brown noted that a key consideration in his work has been that dislocations and impingement/subluxation events need to be addressed fundamentally as kinetic phenomena, ie, that forces, moments, and local stresses in the tissues and materials involved are what matter clinically. Although interrelated with traditional kinematic (geometric) factors such as range-of-motion, these kinetic factors govern whether a given impinging implant will or will not dislocate, and whether tissues and/or implant components will or will not be harmed during a given impingement event. Including kinetic considerations, however, greatly increases the complexity and difficulty of quantifying impingement/dislocation events.
The principal laboratory research methodology Dr. Brown and his colleagues used has been finite element analysis (FEA). “An FEA approach to THA impingement/dislocation held the attraction that once appropriate investments were made in model development and validation, individual variables or combinations of variables could be investigated systematically and efficiently, in virtually unlimited depth and detail,” he wrote.
Building a database
To gather physiologically realistic input data, the researchers undertook studies to determine muscle forces and articular contact force at the hip. A motion analysis system was used to record the kinematics of age-matched individuals as they executed a battery of dislocation-prone maneuvers such as leg-crossing, rising from a low seat, or bending to tie a shoe.
Based on this physiologic data, FEA analysis enabled them to undertake parametric studies of implant design factors, component surgical positioning factors, and patient motion challenge factors surrounding THA dislocation.
The authors admit that reliably predicting which patients will experience instability-related problems is unrealistic “in the foreseeable future.” But they believe that their research will help physicians make informed clinical judgments to reduce the likelihood of these problems.
Dr. Brown’s future research will focus on improving the current model through a series of specific steps and ongoing internal technical upgrades. He also hopes to develop a better knowledge base of the numbers and frequencies of impingement/dislocation challenge events that THA patients encounter in daily life. Finally, he hopes to develop a basis for integrating multiple concurrent risk factors so that reasonable trade-offs can be made between directly competing individual variables.
“Because so many of the issues related to these implant complications are biomechanical, information derived from reliable biomechanical models can help surgeons make better-informed decisions. Better decisions may reduce the likelihood that patients who receive implants will experience problems,” said Dr. Brown.
Disclosure information: Dr. Brown—Journal of Orthopaedics and Traumatology, Journal of Bone and Joint Surgery–American; Dr. Callaghan—DePuy, A Johnson & Johnson Company; Wolters Kluwer Health Lippincott Williams & Wilkins. Mr. Elkins and Dr. Pedersen reported no conflicts. Funding was received from NIH/NIAMS, the U.S. Department of Veteran Affairs, and DePuy, A Johnson & Johnson Company.
Terry Stanton is the senior science writer for AAOS Now.
Madeleine Lovette is the communications specialist in the AAOS office of government relations.
- Although impingement, subluxation, and dislocation are significant events in patients with hip implants, their biomechanics have not been thoroughly investigated.
- Predicting which patients will experience instability-related problems is not possible due to the number of variables and unknowns.
- Biomechanical models based on finite element analysis can provide information to enhance surgical and patient decision-making.
- Both kinetic and kinematic factors affect whether an impinging implant will dislocate.