Monday, September 22, 2014

Resurfacing humeral arthroplasty can cause bone loss beneath the component

Stress-shielding induced bone remodelling in cementless shoulder resurfacing arthroplasty: A finite element analysis and in-vivo results

These authors explore the concern that cementless surface replacement arthroplasty may result in stress shielding and bone remodelling beneath the prosthesis. They studied bone remodelling using 3-dimensional finite element analysis (FEA) as well as evaluation of contact radiographs from human implant retrievals. 
FEA included one native humerus model with a normal and one with a reduced bone stock quality. The compressive strains were evaluated before and after virtual resurfacing prosthesis implantations.

They also studied the bone remodelling and stress-shielding pattern of 8 human cementless surface replacement arthroplasty retrievals.

FEA revealed for both bone stock models increased compressive strains at the stem and outer implant rim for both cementless surface replacement arthroplasty designs indicating an increased bone formation at those locations. Unloading of the bone was seen for both designs under the central implant shell indicating high bone resorption. Those effects appeared more pronounced for the reduced than for the normal bone stock model. 

These assumptions of the FEA were confirmed in the cementless surface replacement arthroplasty retrieval analysis which showed bone apposition at the outer implant rim and stems with highly reduced bone stock below the central implant shell. Overall, clear signs of stress shielding were observed for cementless surface replacement arthroplasty in the in-vitro FEA and human retrieval analysis. Especially beneath the central part of the cementless surface replacement arthroplasty the bone stock was highly resorbed. 

Comment: As pointed out in our post from two days ago, resurfacing humeral hemiarthroplasty has been proposed as a more conservative approach to managing shoulder arthritis, but it has the disadvantages of (1) non addressing the glenoid side of glenohumeral arthritis, (2) blocking access to the glenoid if a glenoid component is considered, and (3) making it difficult to detect if the humeral component is subsiding. This article adds 'stress shielding' and resulting loss of the supporting bone as a fourth concern.