Experimental Prediction of Contact Area in Hip Replacement and Hemi- Arthroplasty

Transcription

1 Experimental Prediction of Contact Area in Hip Replacement and Hemi- Arthroplasty Qianqian Wang, John Fisher, Sophie Williams. Institute of Medical and Biological Engineering, School of Mechanical Engineering, University of Leeds, Leeds, United Kingdom. Disclosures: Q. Wang: None. J. Fisher: 1; DePuy Synthes Joint Reconstruction. 2; DePuy Synthes Joint Reconstruction. 3A; Tissue Regenix plc. 3B; DePuy Synthes Joint Reconstruction, Invibio, Tissue Regenix plc. 4; Tissue Regenix plc. 5; DePuy Synthes Joint Reconstruction. S. Williams: 3B; DePuy Synthes. Introduction: The human hip is subjected to several times body weight during daily activities, experiencing excessive loads and motions. Techniques have been established to assess joint contact mechanics in vitro [1-4], but few methods have been applied to joints with high conformity, such as the hip. Compared to computational modelling of contact areas, discrepancies have been reported due to finite thickness and stiffness of additional media inserted in the joint clearance [5, 6]. Our studies considering the influence of biomechanical factors on the natural hip joint revealed a need for a method to experimentally assess contact area reliably. This study aimed to develop and validate a new method for use in total hip replacement (THR) and hip hemiarthroplasty (HA). In all combinations the influence of load and joint clearance on contact mechanics was investigated. Methods: Two hip replacements (Table 1, supplied by DePuy Synthes, UK) were used for contact area measurement (n = 3) and results validated against analytical results derived from Hertzian contact theory [7]. Additionally, five acetabula from pigs (age 24~26 weeks) were prepared for HA studies. The associated porcine femoral heads were measured geometrically and had mean diameter of 37.3 mm (± 1.1 mm in deviation). Cobalt chrome (Co-Cr) heads in diameter of 37 mm, 35 mm and 32 mm were paired to produce diametral clearances of 0.3 mm, 2 mm and 5 mm respectively. The natural acetabulum was cemented in a cup holder at an inclination angle of 35 and allowed to rotate around an axis perpendicular to the inclined surface of the cup holder (Figure 1a). The zero position of this rotation was defined as the margin of the acetabular notch facing upwards. Two angles (- 15 and -30 where negative denotes anti-clockwise rotation) were considered to simulate the anatomical orientation of acetabulum in the pelvis. In both the THR and HA studies, a coating & squeeze technique was employed using a liquid polymer (Microset Products Ltd., UK, model 101RF) as a media to coat the femoral head, when subjected to a load, the media was squeezed out of the area in contact. Head and acetabulum pairs were fixed into a materials testing machine (Instron model 3365, Instron Ltd., UK) in a setup that was the inverse of the anatomical situation, the holder of the acetabular cup was mounted on an XY translating table, and the head was driven downward (Figure 1b). The demand load varied from 50N to 2400N for full prosthetic hip joints, and from 10N to 400N for HA hip joints and ramped up over 10 seconds. The head and the cup showed the contact area and were photographed in each contact configuration. In the post-processing, the 2D contact patch in the photograph of the head was projected onto a spherical surface, which had the same diameter as the head, to give the magnitude of the curved contact area.

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3 Results: The experimental measurements of contact area in THR were compared to analytical results and demonstrated good agreement (Figure 2). A minimum resolution of < 0.5 mm in contact radius was observed at a load of 50 N in hard-on-hard bearings. In the HA study resultant contact area was quantitatively compared with varying loads and rotation angles (Figure 3a). The area in contact increased with the load applied (approximately 200 mm 2, 275 mm 2, 350 mm 2 and 400 mm 2 corresponding to loads of 50 N, 100 N, 200 N and 400 N respectively). Reducing joint clearance increased the contact area (Figure 3b), and decreased the average contact pressure (i.e. the measured area divided by the load applied), e.g. from 2.57 MPa in the case of 5 mm clearance to 0.96 MPa in the case of 0.3 mm clearance at a load of 400 N. Statistical analysis indicated the load and joint clearance to be significant factors affecting the contact area between the acetabular cartilage and the artificial head (p < 0.001

4 in one-way ANOVA for each rotation angle in Figure 3a; p < 0.5 in one-way ANOVA for each demand load in Figure 3b). However, the orientation of the acetabular cup had little effect. For the best matched HA hip joint, the contact patch showed a crescent shape covering the majority of acetabular cartilage surface, and extended in anterior-posterior direction along the acetabular rim externally and the edge of the margin around acetabluar dome internally (Figure 3c).

5 Discussion: The coating & squeeze method has produced reliable and repeatable outcomes in THR and HA hip joints. In terms of area, the accuracy was half of the region minimally detectable in Tekscan sensor model 4400, which had sensel density of 64.1 per cm 2 [8]. A non-linear relationship between contact area and the load was observed in HA hip study. The decaying trend implied that the contact area on the articular cartilage approached the maximum as the load increased. The varying load was found to change the contact mechanics surrounding the acetabular cartilage significantly. The degree of the prosthetic femoral head matching the natural acetabulum has also shown to be important to ease the mechanical environment. In a similar load and HA joint configuration, the contact area given in this study was about 20% larger than that using Fuji film, and was comparable to the computational model by Pawaskar et al [6]. Significance: The coating & squeeze method presented in this study was proved to be reliable to assess contact area experimentally and had clear advantages for high conforming hip joint. The findings in hemi-arthroplasty hip joints imply that the mechanical environment surrounding the articular cartilage is significantly dependent on the degree of the prosthetic

6 femoral head matching the natural acetabulum. Acknowledgments: This study was supported by Engineering and Physical Sciences Research Council in United Kingdom under the grant number EP/G012172/1. References: [1] Ateshian GA, et al., J Biomech 27: [2] Stewart T, et al., Proc Inst Mech Eng - Part H, J Eng Med 209: [3] Black JD, et al., Clin Orthop Relat R 159: [4] Wilson DR, et al., J Biomech 36: [5] Liau JJ, et al., Clin Biomech 16: [6] Pawaskar SS, et al., J Biomech 44: [7] Johnson KL, Cambridge University Press. [8] Tekscan Inc., USA. ORS 2014 Annual Meeting Poster No: 1794

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