The Exeter Hip Replacement was designed by Professor Robin Ling of ..
Rokkum M, Bye K, Hetland KR, Reigstad A. Stem fracture with the Exeter prosthesis. 3 of 27 hips followed for 10 years. Acta orthopaedica Scandinavica. 1995 Oct;66(5):435-9. PubMed PMID: 7484124.
Last Word Archive | New Scientist
Swarts E, Kop A, Jones N, Keogh C, Miller S, Yates P. Microstructural features in fractured high nitrogen stainless steel hip prostheses: A retrieval study of polished, tapered femoral stems. Journal of Biomedical Materials Research Part A. 2008;84A(3):753-60.
Background: Even for clinically successful hip stems such as the Exeter-V40 occasional failures are reported. It has been reported that sub-optimal pre-operative planning, leading to implant undersizing and/or thin cement mantle, can explain such failures. The scope of this study was to investigate whether stem undersizing and a thin cement mantle are sufficient to cause implant loosening. Methods: A comparative in vitro study was designed to compare hip implants prepared with optimal and smaller than optimal stem size. Exeter-V40, a highly polished cemented hip stem, was used in both cases. Tests were carried out simulating 24 years of activity of active hip patients. A multifaceted approach was taken: inducible and permanent micromotions were recorded throughout the test; cement micro-cracks were quantified using dye penetrants and statistically analyzed. Findings: The implants with an optimal stem size withstood the entire mechanical test, with low and stable inducible micromotions and permanent migrations during the test, and with moderate fatigue damage in the cement mantle after test completion. Conversely, the undersized specimens showed large and increasing micromotions, and failed after few loading cycles, because of macroscopic cracks in the proximal part of the cement mantle. While results for the optimal stem size are typical for stable hip stems, those for the undersize stem indicate a critical scenario. Interpretation: These results confirm that even a clinically successful hip prosthesis such as the Exeter-V40 is prone to early loosening if a stem smaller than the optimal size is implanted.
Archaeology News -- ScienceDaily
Another factor leading to potential failure is incorrect stem size. Cristofolini et al compared optimal and undersized Exeter™ V40™ stems in vitro, and recognised that undersizing led to micromotion and failure due to crack formation in the proximal cement mantle. It is not uncommon to find that the size of the femoral canal at the time of surgery will only accept a prosthesis that does not provide enough strength for the load placed on it. In the primary hips in our series, offset was reproduced in all cases and stems were all as large as could be accommodated by the narrow femoral canals present in the patients. While stems were small, they were not felt to be undersized.
The Times & The Sunday Times
We have described a series of fractured Exeter™ stems (three following primary hip and one following revision surgery) that presented to a single institution performing approximately 400 hip arthroplasties a year from a population of 160,000. The three primary hip replacements were considered to have lower neck cuts and the revision hip case had poor proximal support due to bone loss following the first revision procedure. All patients were obese and no stems were considered undersized although stems in the primary hip replacements were small. We believe that failure is multifactorial. The failure of smaller stems in obese patients is worrying and there may be a population of patients at risk of failure in the future.
News and opinion from The Times & The Sunday Times
Cement mantle with gross deficiencies - no cement below the stem, major defects in the mantle, or multiple large voids in the mantle Endpoints - need for revision surgery - clinical failure (a painful arthroplasty) - radiographic failure (loose implant) Mechanisms of Failure Mechanical factors - debonding between stem & cement initially occurs at cement-metal interface - produces high peak stresses in cement mantle proximally & near distal tip of stem - initiates cement cracks esp areas of thin cement / adjacent to mantle defects / pores initiate & propagate Biologic processes - then become more important - particulate polymeric debris gains access to endosteal bone - stimulates foreign-body reaction - bone resorption with fibrous tissue membrane beginning at pseudo-capsule extending along cement-bone interface "Definitely loose" - migration of prosthesis - cement mantle fracture "Probably loose" - continuous radiolucent line at cement-implant "Possibly loose" - radiolucent line cement - bone Autopsy studies Radiolucencies most commonly related to skeletal remodelling - not to the formation of soft-tissue membrane between cement & bone - inner cortex commonly forms adjacent to cement mantle - not distinguishable from cement on Xray - 2nd medullary canal forms between inner & outer cortex - appears as radiolucency on Xray - non-progressive