The Morse Taper is a method used by machinists to reliably join two rotating machine components. The orthopaedic industry has adapted these tapers, under the generic name of Morse tapers, as a means of reliably joining modular components of total joints directly on the operation table.

The principle of the Morse taper is that of the cone in the cone. The trunnion (the male portion) and the bore (the female portion) are both uniformly tapered. When the bore in the femoral head is tapped onto the trunnion of the femoral shaft they come into intimate contact. The conical femoral taper compresses the walls in the bore as it expands. Thus, the stresses (red arrows) inside the materials keep both components fixed together. To get the ball to hold on tight on the trunnion, you tap it onto the femoral trunnion.

Picture: Morse taper, Howmedica’s Precision Osteolock system

The Morse taper allows combining of different materials in one total hip model. It is now possible to combine a ceramic femoral head with a titanium femoral shaft. To appreciate the benefits of the Morse taper, remember that the first ceramic femoral heads were pasted on the stem component with acrylic glue! The main cause of failure in these early ceramic total hips: failure of the glue.

Morse tapers have also some disadvantageous sides.

First, the dimensions of Morse tapers are not standardized, they vary from company to company. Thus the male parts manufactured by company A must not be mixed with the female part manufactured by company B. They may appear identical, but they are not! Total hip system assembled from such components is at great risk to dissociate.

Second, the Morse taper connects two different materials, thus fretting and corrosion can be a problem with Morse tapers. To avoid such problems the surgeon must be sure to have the joining surfaces clean and dry before taping them together. Some taper designs have almost no interlock at all when wet.

Third, Morse tapers joining the ceramic femoral heads with the femoral shafts produce stresses within the ceramic material. These so called hoop stresses can cause catastrophic fractures of the ceramic femoral head if they exceed a certain level. This happens when the products of two different manufacturers are coupled together. It is thus especially important that a ceramic head is always coupled with matched femoral stem manufactured by the same manufacturer. This demand may seem obvious, but it happened at revision operations that the surgeons used new ceramic heads from new manufacturer on the old femoral shaft components, produced by another (old) manufacturer. This was in the past often the cause of ceramic head fractures.

The risk for such happening is great at revision operations. If you have a ceramic total hip that needs revision operation, discuss this detail with your surgeon. (Which TM total hip system have you and which systems stocks the hospital). 

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References:

Friedman RJ et al.: Current Concepts in Orthopaedic Biomaterials and Implant Fixation. J Bone Joint Surg-Am 1993; 75-A: 1086- 1008

Pennock et al.: J Arthroplasty 2002; 17:773 - 8