Advantages of Milling Versus Broaching the Proximal Femur

Abstract

Femoral canal preparation in cementless total hip arthroplasty requires either broaching or milling of the proximal femur to create an osseous envelope for component implantation. The purpose of this article is to discuss the merits of milling the proximal femur for implant placement in primary or revision total hip arthroplasty.

Historically, broaching the femur during cemented hip arthroplasty was developed to properly size the femoral canal and create an adequate cement mantle. Clinical outcome studies have suggested that a cement mantle <2 mm resulted in early loosening and failure of the femoral stem.¹ Broaching or rasping the endosteal canal provided the surgeon with the tactile sensation to reliably size the femoral canal and ensure an adequate cement mantle. As the technology for biologic ingrowth femoral stems was developed, broaching the femoral canal also aided a surgeon in removing nonsupportive cancellous bone and creating an osseous envelope to support the femoral implant and promote intimate contact between the implant and host bone for bony ingrowth.

Since the 1980s, numerous biologic ingrowth implants have been introduced into the orthopedic market with a variety of sizes and shapes. Debate over the optimal size and shape of the femoral component for biologic ingrowth stems continues. In a landmark article, Noble et al² described the available anatomic variations in proximal femoral anatomy. Their cadaver study showed no constant relationship between the size of the femoral canal and the size and shape of the proximal metaphysis. Based upon these anatomic considerations, the concept of “fit and fill” was coined to address how well an ingrowth stem fit into the proximal femur in terms of its contact area with host bone and how well the shape of the prosthesis filled the intramedullary portion of the proximal femur. The theoretical concept of fit and fill translated into optimal biomechanics for load transfer between the implant and host bone by maximizing contact area, translating into the most favorable environment in which to promote bone ingrowth.

Figure 1: The femur is prepared for the implant by milling the proximal calcar bone so that the sleeve will be tightly fitted to host bone.

Because only a finite number of implant sizes with a broaching system exist, many surgeons and engineers began exploring the concept of milling the proximal femur to provide an exact envelope for implant placement. Paul et al³ developed an approach to milling that used a surgical robot (“ROBODOC”) to machine the proximal femur. They compared broaching to robotic milling preparation in human cadavers and found 96% implant perimeter contact with host bone for milled specimens compared to 21% perimeter contact for broached specimens. Another milling implant system developed during this period was the S-ROM total hip prosthesis (DePuy Orthopaedics Inc., Warsaw, Ind). Preparation of the proximal femur is accomplished by a combination of incrementally sized conical reamers and a calcar miller (Figure 1). This system allows for several combinations of cone sizes and calcar sizes to accommodate the array of endosteal canal anatomy. In an unpublished study, a comparison of the S-ROM miller and the robotic miller demonstrated no significant difference in the percentage of perimeter contact with host bone (personal communication with Kim Dwyer, Senior Engineer, DePuy Orthopaedics Inc.).

The increased contact area between the implant and host bone promotes a theoretical advantage of improved bone ingrowth, as well as increased initial implant stability for early weight bearing in milled proximal femurs. However, several clinical studies with different broached implants have not shown a significant clinical or radiographic disparity between these two systems. An early biologic ingrowth stem that used broaches for canal preparation was the Porous Coated Anatomic (PCA) stem (Stryker, Mahwah, NJ). Bojescul et al⁴ reviewed the long-term clinical and radiographic follow-up of the PCA stem. Although they reported a 6% femoral loosening rate at a minimum 15 years postoperatively, the majority of stems loosened due to polyethylene wear and osteolysis rather than mechanical loosening. D’Antonio et al⁵ reported the 10- to 13-year follow-up of the Omnifit hydroxyapatite stem (Stryker). They observed excellent results with a mechanical loosening rate of 0.5%. Both of these stems incorporate an anatomic basis to their design and approximate the fit and fill of a patient’s femoral endosteal canal as close to anatomic as possible. The limitations are related to the finite number of sizes available and the generic shape of the stem.

The Tri-Lock stem (DePuy Orthopaedics Inc.) is another broached stem that gains fixation by wedging along the medial and lateral aspects of the femoral metaphysis. This stem is narrow in the coronal plane and broader in the sagittal plane. Therefore, cortical contact with host bone occurs only at the proximal medial and lateral borders of the stem. Although there are no cadaver studies examining the percentage of perimeter contact with cortical host bone, it is clear from the implant design that maximizing perimeter contact with cortical bone is not the primary goal of fixation. Burt et al⁶ reported on the average 10-year follow-up of this stem design and noted a 2% femoral loosening rate.

Figure 2: The S-ROM system’s modularity provides a reconstructive surgeon with numerous options for re-approximating anatomy and optimizing biomechanics.

The results of the S-ROM prosthesis (Figure 2) were reported by Christie et al⁷ at 4- to 7-years intermediate follow-up. The goal of the S-ROM stem is to maximize contact with host bone at the proximal ingrowth surface. A patient’s femoral canal is independently sized in the diaphysis, the metaphysis, and the calcar regions of the femur. Multiple sleeve combinations are available to match or fit and fill a patient’s anatomic variations. The concept of milling the proximal femur allows a surgeon to identify the stem and sleeve combination that most closely matches a patient’s anatomy and then mill the bone for an exact fit. This process produces a “customized” implant for each patient while maintaining intimate contact between the ingrowth region of the sleeve and host cortical bone. Christie et al⁷ reported only one femoral component failure in their multicenter study.

The clinical and radiographic results comparing femoral stems that use broaching in the preparation of the femoral canal with stems that require milling of the canal to achieve intimate contact with host bone have shown little differences in outcomes. Although the concept of customizing the femoral stem to match a patient’s endosteal anatomy seems intuitive, the published results have not yielded a significant difference as compared with broached stems.

Another emerging aspect of postoperative management includes the use of less invasive incisions and early weight bearing on the operative extremity. During the time period that most published long-term outcomes studies were performed, the investigators did not specifically address early weight bearing. Most outcome studies of biologic ingrowth stems generally limited weight bearing in the early 3- to 6-week postoperative period. A potential advantage of the milled S-ROM hip stem is the possibility of full weight bearing immediately postoperative. Mason et al⁸ examined the radiographic evidence of stem subsidence with the broached Summit (DePuy Orthopaedics Inc.) in patients who were allowed early full weight bearing. They noted 1-2 mm of nonprogressive subsidence in 2% of patients. Presently, no published information on this topic with the S-ROM prosthesis exists, but the author has not observed any evidence of stem subsidence in primary hip arthroplasty (approximately 200 hips) in patients over the past 4 years during which protocol has allowed full weight bearing in the immediate postoperative period.


Figure 3: Preoperative radiograph of a typical osteoarthritic hip. Figure 4: A broach-type implant system will achieve stability, re-establish leg lengths, and provide sufficient offset in this hip. Figure 5: The S-ROM system will achieve stability, re-establish leg lengths, and provide sufficient offset.

The major advantage of milling versus broaching the femoral canal is the ability for a surgeon to predetermine the placement of the femoral prosthesis to recreate a patient’s normal anatomy and hip biomechanics. In a broaching system, the conventional method of stem placement is to seat the stem as far distally in the femoral canal as necessary to ensure adequate contact with cortical bone. Thus, the shape of the proximal femur where this contact occurs along with the selected size of the femoral stem determines the position of the femoral head center. The position of the head center of rotation defines the lateral and vertical offset of the stem. Figures 3 and 4 demonstrate this phenomenon. Figure 3 is a radiograph of a typical osteoarthritic hip with the patient’s anatomic characteristics of lateral offset, vertical offset, diaphyseal size, and metaphyseal size shown. Figure 4 is a representation of a template for a broached stem fitting within the anatomic constraints of the endosteal canal. After selecting the appropriate size femoral component, the hip center of the implanted femur is determined by the shape of the femoral prosthesis. In this figure, a close match of the implant center of rotation and the patient’s native anatomic center of rotation and thus reapproximation of the patient’s vertical offset (leg length) and lateral offset (hip abductor moment arm) is present.

Manufacturers of broached stems have recently addressed the issue of patients with greater lateral offset by providing a high offset version of the component for the same size femoral stem. This high offset stem increases the lateral offset or abductor moment arm of the implant without affecting the position of the seated stem within the femur so that vertical offset or leg length is unaffected. With the S-ROM prosthesis, the desired hip center of rotation is selected first then the proper stem with the proper offset is chosen. Next, the sleeve size that best fits the patient’s metaphysis at the chosen neck resection level is determined. The milling preparation of the proximal femur allows a surgeon to fit the sleeve or customize the placement of the implant based upon a surgeon’s desired placement of the femoral head center of rotation (Figure 5).

Broached stems work well matching a patient’s anatomy within the “normal” population of femoral anatomy. However, as observed by Noble et al,² a range of “normal” femoral metaphyseal and diaphyseal anatomy exists. In the situation shown in Figure 6 where the patient has a significantly valgus orientation of her proximal femur, use of a broached femoral stem will not allow the reconstruction of normal hip biomechanics. When the femoral stem is fully seated against cortical bone, the vertical and lateral offset do not match a patient’s native anatomy (Figure 7). The S-ROM component works well in this situation because the surgeon starts by selecting the position of the hip center and then fits the stem and sleeve to match the patient’s endosteal canal anatomy. Figure 8 demonstrates this with a higher neck cut and use of a larger sleeve to fit the patient’s anatomy. A well-fitted implant with good restoration of biomechanics is the result (Figure 9).

The theoretical advantage of increased contact area with host bone using a milling system does not appear to provide superior clinical results compared to a broaching system. However, issues concerning early weight bearing may favor the milled femoral component as early implant stability becomes more important. Patients with wide variations of femoral canal anatomy may be addressed with the S-ROM milling system to recreate normal hip biomechanics.


Figure 6: Preoperative radiograph a valgus osteoarthritic hip. Figure 7: Achieving stability on host bone while establishing stable biomechanics is difficult with a broach-type implant system. Figure 8: The higher neck cut allowed by S-ROM’s milling system allows the surgeon to achieve a stable hip. Figure 9: Postoperative radiograph shows a well-fitted implant with good restoration of biomechanics.

References

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Bojescul JA, Xenos JS, Callaghan JJ, Savory CG. Results of porous-coated anatomic total hip arthroplasty without cement at fifteen years: a concise follow-up of a previous report. J Bone Joint Surg Am. 2003; 85:1079-1083.
D’Antonio JA, Capello WN, Manley MT, Geesink R. Hydroxyapatite femoral stems for total hip arthroplasty: 10- to 13-year followup. Clin Orthop. 2001; 393:101-111.
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Mason JB, Squire MW, Fehring TK, Odum SM. Comparison of weight-bearing protocol on ingrowth and subsidence of porous-coated tapered stems. Proceedings of the 72nd Annual Meeting of the American Academy of Orthopaedics Surgeons. 2005; 6:357.

Author

Dr BeBoer is from Southern Joint Replacement Institute, Nashville, Tenn.