by Jack Lemons, MS, PhD
Jack Lemons, MS, PhD, is director of Surgical Laboratory Research and Resident Research Program for the Division of Orthopaedic Surgery, University of Alabama at Birmingham, Ala.
A number of strong opinions about orthopaedic total joint arthroplasty (TJA) devices with Morse taper modular connections have been expressed by clinical and engineering investigators. Some have proposed limitations on modularity based on examples of adverse conditions where biodegradation phenomena have resulted in debris which could contribute to: third-body mediated wear of articulating surfaces; tissue responses leading to aseptic loosening; and component disassembly or mechanical fracture. In contrast, other investigators have strongly supported the value of modularity and have shown examples demonstrating stable modular connection that have not generated debris. Supporters of modularity have listed combinations of designs and materials intended to enhance device biocompatibility and longevity.
A question often asked within the clinical community is, "what specifically is a Morse taper and can it provide a stable connection between TJA components?" A Morse taper is defined by the angle that the taper surfaces make relative to the longitudinal axis of the component. Morse taper angles used for TJA generally fall in a range of 2 to 12 degrees and when interpenetrating parts of slightly different sizes or angles are assembled as an interference fit, they lock together. This is because of the high contact stresses that develop along the interface during forced assembly. This can result in cointegration (locking) with material transfer across the zone of contact (cold welds). The degree of fit (interference) is determined by the relative dimensions of the two components (male and female regions) and a design decision to have interference along a specific part of the taper's circumference and length. The area of interference contact must be adequate to maintain integrity under functional (loaded) conditions, while the surface finish of the components must be specific to the physical and mechanical properties of each component's material. Circumferentially-machined microgrooved (residual ridges), micro-textured, ground, selectively roughened, or chemically modified (e.g. nitrides) surfaces have been introduced to enhance interfacial stabilization for specific biomaterials.
Issues related to the cause(s) of biodegradation (or not) along tapers have been investigated by a number of groups. The initial observations of corrosion phenomena along titanium alloy stems and cobalt alloy heads from explanted total hip arthroplasties (THA) resulted in the expression of several opinions about cause-effect relationships. Fretting mediated corrosion and alterations of the local environment (acidity) within these taper interfaces were implicated with suggestions that specific alloy properties (galling) or mixed alloy (galvanic) reactions were critical to the process. Subsequent laboratory investigations plus clinical reports of taper interface fretting-corrosion for junctions between similar composition alloys and junctions of alloys with nonelectrically conducting ceramics have shown that mixed alloy corrosion, per se, was not the cause of this biodegradation. Fretting and fretting-corrosion phenomena were found to be most critical which have been correlated with variabilities of manufacturing tolerances plus design and clinical conditions that introduced larger magnitude interfacial micromotions during function. Enhanced designing, manufacturing, and inspecting procedures are now routine for modular TJA systems, and study reports support a minimization of biodegradation for TJA modular connections.
Several factors are recognized to be important related to modular TJA devices and Morse taper connections. These include the following general opinions.
In summary, it appears that the overall value of taper junctions will be determined through comparisons of advantageous and disadvantageous characteristics of TJA related to their ongoing use. Current trends indicate that orthopaedic surgeons will continue to utilize monoblock and modular TJA devices with differences determined after longer-term clinical applications. Importantly, current designs and manufacturing practices have evolved and resolution of this issue should not depend upon debris generation due to fretting-corrosion, since this is understood and can be minimized. Issues related to modularity or not, hopefully, will be resolved before the year 2020.
Jack Lemons, MS, PhD, is a consultant to the Committee on Biomedical Engineering.