Image Quiz

An Unanticipated Postoperative Radiograph1 (continued)

Answer: Dissociation of a Morse-Taper stemmed tibial component following revision total knee arthroplasty.

Fig. 1 Anteroposterior (Fig. 1-A) and lateral (Fig. 1-B) radiographs demonstrate dissociation of the stem from the tibial baseplate just after implantation.

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Discussion

Modular orthopaedic implants involving conical press-fit fixation (a Morse taper) include a tapered shank that fits into a socket of the corresponding taper, providing accurate alignment and frictional fixation. Under normal loading, the cold weld achieved between the two surfaces is stable, and dissociation between the two metal surfaces can be achieved only with excessive force. Consequently, failure of such components is rare. Previous studies have demonstrated that contamination by fluids, galvanic corrosion, and trauma are associated with failure of the taper lock.

A modular junction can be located at or near the center of motion or distal to it. In most reports of Morse taper dissociation, the modular junction was at either the femoral or the humeral head, with disassembly occurring secondary to joint instability. In our patient, the dissociation was away from the axis of joint rotation, and the failure was due not to instability but to immediate interlock failure.

Most orthopaedic implants do not conform to the standard specifications defined by the American Standards for Morse tapers. Orthopaedic tapers have cone angles of between 4° and 6°, whereas the standard industrial Morse taper has a cone angle of between 2° and 3°. In addition, because of anatomical considerations, the socket and shank lengths used in orthopaedic implants are shorter than those of conventional industrial Morse tapers. Despite these variances, the construct is designed to be stronger than any potential in vivo distraction force. The Coordinate knee system has a 4° taper of cobalt-chromium, and disimpaction of this taper has not been reported previously, to our knowledge. Although there are no specific disimpaction data regarding this particular system, distraction forces of >431 kg (950 lb) in femoral components made of similar metals and with similarly angled tapers have been reported. It is important to remember that distraction forces are greatly reduced in the presence of any machining mismatch. Independent finite analysis testing of the Morse taper used in our patient revealed a 3-µm mismatch between actual and nominal measurements. This may have been sufficient to cause a decrease in the locking strength of the Morse taper.

Mechanical testing of humeral head components has demonstrated that as little as 0.4 mL of water or blood could prevent the taper from seating correctly at the time of impaction. Blevins et al. hypothesized that fluid contaminant in the bottom of the socket could not be displaced by the precision-fit shank, which thus prevented the taper from obtaining a frictional fit. Because back-table assembly of implants is standard practice in our facility, it is unlikely that blood or saline solution caused contamination of the implant used in our patient.

Polymethylmethacrylate has also been implicated as a contaminant that weakens the cold weld in a Morse taper. Blevins et al. demonstrated that a fine layer of polymethylmethacrylate powder on the shank can lower the dissociation force by >10%. Although polymethylmethacrylate is routinely mixed on the back table in a vacuum, it is possible that some powder escaped when the package was opened.

The ex vivo impaction force, the angle of impaction, and the surface on which the impaction occurred are all variables that could be implicated in taper dissociation. Two blows with a mallet, which was used to seat the taper in this case, have been shown to be sufficient to obtain a correct taper lock, and it has been shown that additional blows do not increase the dissociation force. The dissociation force has also been found to be unaffected by the angle of impaction and the surface on which the impaction occurs. Thus, these variables are not factors in the dissociation that occurred in our patient.

We propose that the mechanism responsible for dissociation of the taper was most likely related to the mallet impaction of the tibial component into the bone. The tolerances of the shank and socket interface may have been insufficient to prevent the weld from breaking under an axial load applied with the tip of the intramedullary stem relatively unsupported in the tibial diaphysis or with the tip slightly off-axis from contact with the inner cortex. Off-axis loading has previously been implicated as a cause of increased micromotion on the taper, particularly in the presence of manufacturing tolerances, as shown during simulated in vivo testing.

Our conclusion, therefore, is that forces placed on the prosthesis at implantation are far greater than the physiological forces imposed by activities of daily living, and that any off-axis loading may compromise the cold weld between the components. It is unclear if the dissociation occurred solely as a result of the impaction force on the unsupported construct or if it occurred in combination with a contaminated taper interface.

Reference

1. Kennedy JG, Kearns SR, Quinlan WB. Dissociation of a Morse-taper stemmed tibial component following revision total knee arthroplasty: a case report. J Bone Joint Surg Am. 2003;85:536-8.