Stem Design and Fixation

, Paul D. Siney¹ and Patricia A. Fleming¹

(1)

The John Charnley Research Institute Wrightington Hospital, Wigan, Lancashire, UK

Stem design and its use in total hip arthroplasty have evolved from treatment of fractures of the neck of the femur. Limited exposure in an elderly frail patient, failure to achieve fracture reduction or fixation, demanded a more definitive treatment – a hemiarthroplasty. The exposure was adequate for the purpose; the acetabulum was not exposed; femoral head served to gauge the size. Access to the medullary canal was limited through the neck of the femur, merely for the insertion of the hemiarthroplasty. Cement was not used.

Addition of acrylic cement for stem fixation was considered merely a minor aspect of the procedure. It was at this stage that complications, attributed to the use of acrylic cement, were published. The explanation, in retrospect, is obvious – large medullary canal, fatty marrow, cement trapping air and fat, stem acting as a piston injecting the medullary contents into the patient’s circulation. With the increasing understanding of the function of the stem in the mechanics of total hip arthroplasty the emphasis changed from “replacement” to “reconstruction” of hip mechanics. Exposure demanded a better view of the acetabulum and the access to the medullary canal, not through the neck of the femur, but through the piriform fossa. Longer follow-up, in patients with osteoarthritis, revealed new problems. Stem loosening and fracture were the new complications. They served as a source of information for further developments. It is probably correct to suggest that it was the stem fracture, “an event”: rather than the stem loosening, “a process”: that has made a bigger contribution to our understanding of the function of the stem. This has led to the changes of stem design, materials, methods of manufacture, and the technique of stem fixation.

Stem Design

Stem design and the method of fixation are better understood if an attempt is made to answer the basic question: What is the rationale dictating the use of a stemmed femoral component? It serves, primarily, as an anchorage and support for the extra-medullary portion of the stem. In its proximal part it must resist torsion, in its distal part it must prevent tilting. A stemmed femoral component can be best considered as having three parts, the head, the neck and the shaft. Each one serves a different function and may, therefore, be subject to variations in materials and design, yet together they form an integral part both of the stem and of the arthroplasty.

The Head

The two parameters are the size and the materials used.

The size would be related to the frictional torque of the design, the perceived concept of stability and wear characteristics of the materials used. The materials would be related primarily to strength, wear and the ultimate fate of wear products within the body environment. In any modular system the head-neck assembly would have to be considered.

The Neck

The dimensions of interest are: strength and diameter; hence head-neck ratio and therefore the range of movements, in a plane, before impingement; the neck length and therefore its contribution to limb length; and the neck-shaft angle which would determine the offset of a particular design.

The Shaft

It is the understanding of the function of the intramedullary portion, in the function of the arthroplasty as a whole, that is the essential aspect both in the stem design and the methods of stem fixation.

Stem Design and Methods of Fixation

The Shaft

The intramedullary part serves primarily as an anchorage and a support for the extramedullary part: the head and the neck. In clinical practice two methods are used to achieve that objective: a single stem-femur complex “interference fit” or male/female tapers engaging under load “taper fit”.

Interference Fit: A Single Stem-Femur Complex

The aim, both of the stem design and the surgical technique, is to achieve an integral stem-femur unit. To increase the surface area of the stem-bone contact, the surface of the stem is bead or porous coated. To encourage bony on-growth/in-growth the surface is treated; as in hydroxyapatite designs.

The instrumentation and the surgical technique are aimed at achieving an interference fit between the implant and the endosteum. The final step, in stem fixation at surgery, is a combination of rasp effect of the advancing stem, interference-fit, elasticity of the femur, strain generation and relaxation. The immediate fixation is a combination of interference fit and the elasticity of the femur. The quality of that fixation is governed by “the last hammer blow” and is determined by the contact at the asperities. The long-term anticipated quality of fixation is by bony on-growth or in-growth – osseointegration. Osseointegration is the characteristic of the skeleton. Thus the implant is controlled by the designer and the manufacturer, while the ultimate mode of fixation – osseointegration is “under the control” of the patient’s skeleton. Surgical technique now plays a minor role and is largely replaced by the implant – a commodity that can be sold at a profit.

The advantage is an undemanding simplicity. The disadvantage of the method is the inability to judge the quality of fixation at surgery – hence fractures of the femur, at surgery, may result. If osseointegration is achieved the problems of stem extraction, at revision, will have to be addressed.

Taper Fit: Male and Female Tapers Engaging Under Load

The method takes the advantage of the long established engineering principle the male and female tapers engaging under load. When the system becomes load-bearing it must allow a slip of the male taper, the stem, within the female taper, the cement.

The Male Taper: The Stem

For the stem to be allowed to slip within the cement mantle, it must have a polished surface, continuous taper, it must not be supported distally.

The Female Taper: The Cement

The cement must be of good quality and well supported by strong cancellous bone over the largest possible area.

Surgical Technique

The surgical technique is the essential element of the method.

The objective is to offer the largest possible area of contact between cement and bone and thus reduce pressure per unit area and to ensure the strongest possible bone-cement interface in order to resist both hoop and torsional loads while avoiding proximal strain shielding of the femur. It must also allow a limited slip of the stem within the cement mantle.

The method is highly design and technique dependant. Stem design can no longer be a replacement for the surgical technique, and the technique is not a commodity that can be sold at a profit.

It cannot, therefore, come as a surprise that in clinical practice there is a move away from the demanding surgical technique towards the “more forgiving” implants, as well as “cementless” fixation of components. (A “forgiving” implant is a rather unfortunate term; it admits transgression without promise of repentance.) “Cementless” fixation on the other hand does not define a method of fixation except by stating what it is not. An addition of “porous” or “HA” coating merely describes the surface finish of the implant and not the method of fixation.

Stem Design: The Taper

The geometry of the medullary canal presents a range of shapes and dimensions. They may be graphically and unconventionally described by the capital letters U, V, Y (Figs. 20.1, 20.2, and 20.3). (Though clearly this can only be a rough scheme and any variation will demand a combination of the letters; an interesting but not a particularly helpful exercise). The surgery of total hip arthroplasty demands a tapered stem irrespective of these variations. It is here that the benefit of the acrylic cement becomes so obvious. It can provide of the whole range of “custom built” individually intimate shapes and sizes with the largest possible contact area, while the stem can now be considered as a separate entity within the cement mantle – a taper fit. (No such contact is possible with a simple interference, “cementless”, fit). Although the concept is one of a simple wedge, the dimensions of the medullary canal and the function of the hip demand a more complex design – a wedge with an offset.

Figs. 20.1, 20.2 and 20.3

The geometry of the medullary canal presented graphically and unconventionally described by the capital letters U, V, Y

The Taper

The taper must allow a predictable, limited slip of the stem within the cement. The stem must also be bulky enough to resist torsion and its effects; loosening and fracture and also allow transfer of the load to the most proximal part of the femur. The dimensions of the taper are primarily governed by the length of the shaft of the stem.

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