S268 Presentation O-265 Knee Stability

THE IMPORTANCE OF CONTACT STRESS IN KNEE HEMIARTHROPLASTY DESIGN: A TRIBOLOGICAL

SIMULATION

Laura McCann (1), Itoro Udofia (1), Eileen Ingham (1) Zhongmin Jin (1), John Fisher (1) 1. Institute of Medical and Biological Engineering, University of Leeds, U.K.

Introduction An alternative type of joint replacement for the young, active patient is hemiarthroplasty, which preserves bone stock for future revision surgery. However, this treatment has had varied success to date [Amstutz, 1994; Sisto, 2005]. The tribological regime in both healthy and those replaced with a hemiarthroplasty is very complex. The mechanisms of wear and degradation of AC are not completely understood. The aim of this study was to investigate the effects of contact stress on friction and wear of articular cartilage, using a tribological simulation of the medial compartment of a bovine knee. Materials and Methods A pendulum friction simulator was used to apply physiological loads and motion, in this tribological simulation of the medial compartment of the natural knee. Bovine medial femoral condyles (18 month old and with approximate radius of 38-42 mm) were articulated against tibial surfaces or stainless steel plates of different conformity to create different stress conditions. Several simulations were carried out as shown in table 1. Simulation Tibial Counterface Natural Tibia +meniscus intact

(AC-AC +meniscus) Meniscectomy Tibia (meniscus removed)

(AC-AC no meniscus) Hemi (flat) Flat SS plate – AC-SSFP Hemi (50mm radius)

50mm conforming plate AC-SSCP50

Hemi (100mm radius)

100mm conforming plate AC-SSCP100

Table 1:Five different tibial counterfaces were used simulating the natural, meniscectomised and hemiarthroplasty-replaced joint. Each simulation was tested at high (1000N) and low (250N) load. The lubricant used was 25% bovine serum in saline. The loading and motion profiles were derived from BS 14243-3:2004. A dynamic axial load was applied (peak loads are referred to in this paper). A motion amplitude between -10º - 13º was used, with a frequency of 1Hz. The coefficient of friction ( ) was measured through a piezoelectric transducer. Contact stresses between bearing surfaces were measured using pressure sensitive Fuji Film and a spectrodensitometer. Results The lowest contact stress value was found for AC-AC+meniscus at the lower loading condition. When the load was increased using this bearing the

friction and contact stress also increased. When the meniscus was removed to simulate a meniscectomy the contact stress and friction increased. The contact stress for the meniscectomised model at low load was even greater than that of the natural bearing at high load.

Figure 1: Average coefficient of friction versus average maximum contact stress for each bearing at high (red) and low (blue) load: The SSFP showed the highest friction and contact stress when tested at high load. With increasing conformity the contact stress and friction were reduced. There was no wear found on AC-AC+meniscus specimen at either load condition, little to no wear on the AC-AC no meniscus or AC_SSCP specimen at low load. At higher loads these showed surface fibrillation, which was more severe for the 100mm radius. For the SSFP specimen, there was surface fibrillation at low load and complete wear through to the underlying bone at high load. Discussion At contact stresses 8MPa there was mild (surface fibrillation) to no wear but it became more severe at higher contact stresses. This investigation showed the importance of contact stress, which depends on biomaterial type, joint structure and conformity in the design of spacer implants or hemiarthroplasty. This study has shown how the biphasic nature of cartilage reduced contact stress and friction in comparison to SS, that meniscectomy increases contact stress and friction and how increased conformity reduces contact stress. Acknowledgement: Funding provided by EPSRC DePuy International References Amstutz et al, Bone Joint Surg Br, 73(3):423-427, 1994. Sisto et al, Bone Joint Surg,87:1706-1711, 2005.

Journal of Biomechanics 41(S1) 16th ESB Congress, Oral Presentations, Wednesday 9 July 2008