Effects of resection thickness on mechanics of resurfaced patellae

Abstract

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.

Introduction

Early total knee replacement (TKR) procedures with resurfaced patellae reported high incidence of patellofemoral (PF) complications, contributing to up to 50% of all revisions (Brick and Scott, 1988). However, improvements in component design and materials have reduced the incidence of revisions due to PF issues. Recent studies have reported incidence of anterior patellar pain of 12% and patellar fracture rates from 0.68% to 5.2% in the TKR population (Helmy et al., 2003, Dalury and Dennis, 2003, Seijas et al., 2009). While occurrence of patellar fracture is relative low, revision surgeries have reported poor outcomes with high complication rates and reoperation (Ortiguera and Berry, 2002), leaving patellar fracture as a significant concern for the comfort and functionality of the TKR population. Patellar resection thickness has been cited as a contributor to patellar fracture, anterior knee pain and extensor mechanism efficiency (Seo et al., 2012), however, the influence of resection thickness on patellar mechanics remains unclear.

During TKR, surgeons typically aim to restore pre-operative patellar thickness, with reports suggesting that this may aid in preserving the natural mechanics of the joint (Hsu et al., 1996). There are a number of concerns associated with resection of the patellar bone that may potentially lead to complications of the PF joint. Overstuffing of the PF joint may induce ligament and muscle tightness as well as limit knee flexion (Marmor, 1988, Mihalko et al., 2006), while excessive resection of the native patellar bone stock may increase strain and the risk of patellar fracture in the remaining patellar bone (Goldstein et al., 1986, Reuben et al., 1991), increase the risk of crepitation (Hoops et al., 2012), and reduce the quadriceps muscle moment arm, leading to a decrease in extension efficiency (Mountney et al., 2008). Studies have recommended against resected bone of less than 15 mm due to increased patellar bone strain at increased resection depths (Reuben et al., 1991), while others have reported no difference in clinical outcome between groups with bone remnants of less 12 mm and those with bone remnants of greater than 12 mm (Koh et al., 2002).

While clinical studies serve to identify factors which differentiate between surgical outcomes based on retrospective analysis, this necessitates high-volume datasets with numerous confounding factors which make it difficult or infeasible to identify the impact of a specific surgical decision, such as patellar bone resection thickness, on patellar mechanics. In vitro and in silico studies have the advantage of evaluating such factors in a controlled environment, isolated from additional sources of variability. A number of cadaveric studies have sought to investigate the effect of patellar resection thickness on patellar mechanics (using patellar strain as a surrogate measure for risk of patellar facture and anterior knee pain), with studies measuring strain on the anterior surface using uniaxial strain gauges reporting an increase in superficial strain proportional to the amount of bone removed (Wulff and Incavo, 2000, Lie et al., 2005, Reuben et al., 1991).

Recent computational modeling work has developed subject-specific finite element (FE) PF models with heterogeneous patellar bone material properties mapped from computed tomography (CT) images (Fitzpatrick et al., 2011). These models have been used to compare patellar mechanics in natural, implanted and unresurfaced conditions (Fitzpatrick and Rullkoetter, 2012). To date, however, they have not been used assess the influence of surgical decisions, such as patellar resection thickness, on mechanics of the patella. The objective of the current study was to determine how patellar resection thickness impacts patellofemoral mechanics, including kinematics and strain in the bone, and hence provide insight into the risk of fracture and anterior knee pain.