The Effects of Cup Compression on Trunion Micromotion in Metal-on-Metal THA Designs

Introduction

Recent retrieval studies and registry reports have demonstrated an alarming incidence of early failure of metal-on-metal THR. This appears to be due to fretting and corrosion at the taper junction (trunnion) between the neck and large diameter heads in metal-on-metal hip implants. It has been proposed that designs with lower bearing clearances and greater cup flexibility deform during implantation leading to increased frictional torque and micromotion at the head-neck taper junction. Small movements at the trunnion may suggest elastic deformation, but large movements may suggest slippage at the friction interface. This study was conducted using retrieved metal-on-metal components to test the hypotheses that: 1. Cup deformation through localized compression leads to increased bearing torque, and 2. Increased torques generated in large head metal-on-metal bearings cause motion of the head-neck taper junction.

Materials and Methods

Nine metal-on-metal hip implants were received from a national joint retrieval service and tested in a mechanical testing machine. The components were of three different designs (ASR, BHR, and Durom) and ranged in diameter from 42–54 mm. A custom jig was constructed to generate controlled radial compression at opposite points on the rim of an acetabular component. The jig was positioned inverted to the normal anatomical position and was angled to simulate the anatomical orientation of the cup (35° inclination, 10° anteversion). With the exception of an initial compression load of 100N, the cups were compressed at 200N intervals to a maximum of 2000N. Three trials at each cup compression load were performed. The torque developed about the trunnion axis was measured as the head articulated through a motion arc of 60° and the friction factor was calculated. Head–neck micromotion was continuously monitored using a non-displacement inductive transducer. Changes in micromotion from the 100N compression load were calculated.

Results

With increasing cup compression loads, higher bearing torques were observed (R² = 0.191; p < 0.001). Higher bearing torques in turn showed higher levels of trunnion movement (R² = 0.555; p < 0.001). (Figure 1) Two cups showed stable bearing torques (range: 2.32 Nm to 2.49 Nm) and trunnion movement (range: −1.13 μm to 0.82 μm). Three cups showed increasing torques (range: 2.35 Nm to 4.57 Nm) and trunnion movement (range: −2.20 Nm to 6.46 Nm) with increased compression loads while four cups responded to increased compression loads with jumps in torque (range: 2.37 Nm to 5.55 Nm) and trunnion movement (range: −1.39 μm to 12.56 μm). The latter four cups experienced jumps in torque and trunnion movement at compression loads greater than 1000N. (Figures 2 and 3)

Conclusions

1. 1.

   Increased torque as a result of cup compression leads to increased motion at the head-neck junction.

2. 2.

   Cup design may contribute to the degree of trunnion movement due to increased bearing torque via cup compression, as higher cup compression loads do not necessarily correspond to higher torques.

3. 3.

   Cups in which increased flexibility is not offset by large bearing clearances are at risk for binding of the head when implanted in rigid acetabula in which compressive loads can range up to 1800N.