Raymond H. Kim

A statistical finite element model of the knee accounting for shape and alignment variability

By characterizing anatomical differences in size and shape between subjects, statistical shape models enable population-based evaluations in biomechanics. Statistical models have largely focused on individual bones with application to implant sizing, bone fracture and osteoarthritis; however, in joint mechanics applications, the statistical models must consider the geometry of multiple structures of a joint and their relative position. Accordingly, the objectives of this study were to develop a statistical shape and alignment modeling (SSAM) approach to characterize the intersubject variability in bone morphology and alignment for the structures of the knee, to demonstrate the statistical models ability to describe variability in a training set and to generate realistic instances for use in finite element evaluation of joint mechanics. The statistical model included representations of the bone and cartilage for the femur, tibia and patella from magnetic resonance images and relative alignment of the structures at a known, loaded position in an experimental knee simulator for a training set of 20 specimens. The statistical model described relationships or modes of variation in shape and relative alignment of the knee structures. By generating new virtual subjects with physiologically realistic knee anatomy, the modeling approach can efficiently perform investigations into joint mechanics and implant design which benefit from population-based considerations.

Specimen-specific modeling of hip fracture pattern and repair

Hip fracture remains a major health problem for the elderly. Clinical studies have assessed fracture risk based on bone quality in the aging population and cadaveric testing has quantified bone strength and fracture loads. Prior modeling has primarily focused on quantifying the strain distribution in bone as an indicator of fracture risk. Recent advances in the extended finite element method (XFEM) enable prediction of the initiation and propagation of cracks without requiring a priori knowledge of the crack path. Accordingly, the objectives of this study were to predict femoral fracture in specimen-specific models using the XFEM approach, to perform one-to-one comparisons of predicted and in vitro fracture patterns, and to develop a framework to assess the mechanics and load transfer in the fractured femur when it is repaired with an osteosynthesis implant. Five specimen-specific femur models were developed from in vitro experiments under a simulated stance loading condition. Predicted fracture patterns closely matched the in vitro patterns; however, predictions of fracture load differed by approximately 50% due to sensitivity to local material properties. Specimen-specific intertrochanteric fractures were induced by subjecting the femur models to a sideways fall and repaired with a contemporary implant. Under a post-surgical stance loading, model-predicted load sharing between the implant and bone across the fracture surface varied from 59%:41% to 89%:11%, underscoring the importance of considering anatomic and fracture variability in the evaluation of implants. XFEM modeling shows potential as a macro-level analysis enabling fracture investigations of clinical cohorts, including at-risk groups, and the design of robust implants.

Technical Challenges of Total Knee Arthroplasty in Skeletal Dysplasia

BackgroundTotal knee arthroplasty (TKA) in patients with skeletal dysplasias is particularly challenging as a result of the anatomic variances and substantial bony deformities. Little has been written regarding technical considerations that should be made when performing TKA in skeletal dysplasia.Questions/purposesWe describe special operative considerations that must be made when performing TKA on patients with skeletal dysplasia, including implant selection and ligamentous balancing.Patients and MethodsWe retrospectively reviewed 12 TKAs in eight patients with varying degrees of deformity (ranging from 30° of varus to 45° of valgus) secondary to three types of skeletal dysplasias: multiple hereditary exostosis, achondroplasia, and osteogenesis imperfecta. Clinical notes, operative records, and radiographic data were reviewed. Minimum followup was 1xa0year (average, 4xa0years; range, 1–10xa0years).ResultsWe used customized implants in three of the 12 knees. Constrained tibial inserts were used in five knees. All 12 knees underwent releases (soft tissue or epicondylar osteotomy) to address gap balancing or patellar tracking. Average Knee Society scores improved from 35.9 preoperatively to 82.9 postoperatively and average function scores improved from 47.9 preoperatively to 96.7 postoperatively. Complications included two transient peroneal nerve palsies.ConclusionsSpecial considerations must be made with regard to implant selection and ligamentous balancing as a result of the unusual anatomy and deformities that accompany skeletal dysplasia, but the short-term clinical results reveal consistent improvements in pain and function.Level of EvidenceLevel IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Effects of resection thickness on mechanics of resurfaced patellae.

Patellar resection thickness during total knee replacement (TKR) has been cited as a contributor to patellar fracture, anterior knee pain and quadriceps efficiency; however, optimal thickness required to minimize clinical complications remains unclear. The objectives of the current study were to determine how patellar resection thickness and bone quality impacts patellar bone strain, kinematics, and quadriceps efficiency. A series of specimen-specific finite element models of the knee joint with distributed patellar bone material properties were developed. Each specimen was virtually implanted with a TKR system. Each specimen was analyzed with patellar bone resected to thicknesses which varied from 9 to 14 mm. Simulations with reduced modulus bone were also performed. Each model perturbation was evaluated during a dynamic squat cycle, and bone strain, quadriceps force and six-degree-of-freedom kinematics were predicted. Highest peak bone strain was predicted in the thinnest patellae, indicating greatest risk of patellar fracture; highest median bone strain was predicted in the thickest patellae. Consistent differences in quadriceps efficiency were predicted; in early flexion the thickest patellae required the least quadriceps force. Greater sagittal plane tilt was observed for the thinnest patellae. Reduced modulus models (50% lower modulus) demonstrated an increase in peak bone strain of up to seven times the original modulus models. Understanding the complex interactions between patellar resection thickness, muscle requirements, kinematics, bone quality, and bone property distribution may aid in developing an understanding of which patients are most at risk from patellar fracture and anterior knee pain and how best to treat individuals to reduce potential complications.

Evaluation of Total Knee Arthroplasty Performed With and Without Computer Navigation: A Bilateral Total Knee Arthroplasty Study

Sequential bilateral total knee arthroplasty performed on 54 patients utilizing navigation (CAS-TKA) in one knee and traditional instrumentation (T-TKA) in the contralateral knee was reviewed at a mean follow-up duration of 2.5years. There were no differences with regard to KSS, ROM, postoperative anatomic alignment, mechanical axis, or tibial angle. There was a statistically significant decrease in outliers for the CAS-TKA group with respect to anatomic alignment (3.7% vs. 17.0%, P=0.024), mechanical axis (6.1 vs. 20.4%, P=0.037) and tibial component alignment (0% vs. 7.5%, P=0.042). There is no apparent benefit of CAS-TKA with regards to KSS, ROM, or alignment in the hands of fellowship-trained total joint specialists. The clinical relevance of reduced outliers in the CAS-TKA group is unknown with the current follow-up interval.

Can a high-flexion total knee arthroplasty relieve pain and restore function without premature failure?

BackgroundHigh-flexion TKA prostheses are designed to improve flexion and clinical outcomes. Increased knee flexion can increase implant loads and fixation stresses, creating concerns of premature failure. Whether these goals can be achieved without premature failures is unclear.Questions/purposesWe assessed pain relief, knee motion, function, incidence of premature failure, and radiographic appearance in patients with a mobile-bearing high-flexion TKA and determined whether preoperative knee flexion affects postoperative knee flexion.Patients and MethodsWe prospectively followed all 142 patients implanted with 154 mobile-bearing high-flexion TKAs between 2004 and 2007. We obtained Knee Society scores (KSS) and assessed radiographs for loosening. Minimum followup was 24xa0months (mean, 46xa0months; range, 24–79xa0months).ResultsAverage knee flexion improved from 123° to 129°. Patients with preoperative flexion of 100° to 120° had a greater postoperative flexion increase (mean, 13°; range, 114°–126°) than patients with preoperative flexion of greater than 120° (mean, 3.0°; range, 128°–131°). The mean KSS improved from 41 to 95 postoperatively. Patients with preoperative flexion of less than 120° had a greater improvement in KSS (62 versus 48). Posterior femoral radiolucent lines were observed in 43% without evidence of prosthetic loosening.ConclusionsOur data were similar to those reported in patients implanted with traditional and other designs of high-flexion TKA. We found no increased incidence of premature failure, although a higher than expected incidence of posterior femoral radiolucent lines merit continued observation. Patients with less preoperative motion were more likely to benefit from a high-flexion TKA.Level of EvidenceLevel IV, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Control‐matched computational evaluation of tendo‐femoral contact in patients with posterior‐stabilized total knee arthroplasty

Painful patellar crepitus is a potential complication in up to 14% of patients following posterior‐stabilized (PS) total knee arthroplasty (TKA). A recent clinical study identified influential patient and surgical variables by comparing a group of crepitus patients with controls matched for age, sex, and body mass index. The purpose of our study was to evaluate effects of variables identified as significant in the clinical study, including patellar ligament length, femoral component flexion, patellar button size, and position of the joint line, on contact between the quadriceps tendon and the PS femoral component. A previously verified finite element model was utilized to estimate tendo‐femoral contact during deep flexion activity. Using discrete perturbations, the computational model confirmed the clinical findings in that an increased patellar ligament length, flexed femoral component, lowered joint line, and larger patellar component all reduced potentially deleterious contact near the intercondylar notch. With the selected level of anatomic and component alignment perturbations, the most influential factor affecting tendo‐femoral contact was patellar ligament length. Three crepitus patients with matched controls were subsequently modeled, and contact with the anterior border of the notch was present in each crepitus patient, but none of the controls. Alternative surgical alignments for these patients were evaluated to improve the potential long‐term outcomes. By characterizing conditions that may lead to painful crepitus, the modeling approach supports clinicians by identifying pre‐surgical indicators and important alignment parameters to control intraoperatively.

10 Avoiding Wound Complications in Total Knee Replacement

Wound complications are difficult problems following a total knee replacement and are best prevented. Careful preoperative evaluation of patients can identify patients with an increased risk of wound complications. Optimizing management of diabetes mellitus, tobacco use, and obesity potentially can decrease risk. Meticulous surgical technique, proper incision selection, and careful hemostasis maximize wound healing. Prolonged wound drainage and skin edge necrosis often necessitate operative intervention. Complex cases may require plastic surgery consultation and soft-tissue transfer procedures.

Patellar component design influences size selection and coverage

BACKGROUNDnPatellofemoral (PF) complications following total knee arthroplasty continue to occur. Outcomes are influenced by implant design, size and alignment in addition to patient factors. The objective of this study was to assess the effect of implant design, specifically round versus oval dome patellar components, on size selected and bony coverage in a population of 100 patients.nnnMETHODSnIntraoperative assessments of patella component size were performed using surgical guides for round and oval designs. Digital images of the resected patellae with and without guides were calibrated and analyzed to measure bony coverage. Lastly, the medial-lateral location of the median ridge was assessed in the native patella and compared to the positioning of the apex of the patellar implants.nnnRESULTSnIn 82% of subjects, a larger oval implant was selected compared to a round. Modest, but statistically significant, differences were observed in selected component coverage of the resected patella: 82.7% for oval versus 80.9% for round. Further, positioning of the apex of oval patellar components reproduced the median ridge of the native patella more consistently than for round components.nnnCONCLUSIONSnThese findings characterized how implant design influenced size selection and coverage in a population of patients. The ability to upsize with oval dome components led to increases in bony coverage and better replication of the median ridge compared to round components. Quantifying the interactions between implant design, sizing and coverage for a current implant system in a population of patients supports surgical decision-making and informs the design of future implants.