The Rationale and Performance of Modularity in Total Hip Arthroplasty

Abstract

Modularity in total hip arthroplasty design is an evolving concept that continues to receive citation in the clinical literature. The advantages of these systems include off-the-shelf flexibility for customizing proximal and distal canal filling, as well as accommodation of difficult situations of femoral deformity and bone loss. Clinical concerns in the application of modular femoral stem hip designs include the maintenance of anatomical stability within the femoral canal component dissociation, structural compromise at metal-metal interconnections due to cyclic microdisplacements (fretting), and the increased potential for metallic-wear debris generation. This article reviews the rationale and performance of typical modular systems.


Figure 1: Evolution of stem design and fixation technique involving monoblock structure: Charnley (DePuy Orthopaedics, Leeds, UK) (A), AML (DePuy Orthopaedics, Warsaw, Ind) (B), and Omnifit (Stryker Orthopaedics, Mahwah, NJ) (C).

More than 350,000 primary and revision total hip arthroplasty procedures are performed annually in the United States alone, demonstrating the enduring success of the low-friction arthroplasty developed by Sir John Charnley.^1,2 Concurrent with the geometrical and material optimization of the cemented, fixed-head Charnley stem were analyses of normal and abnormal hip joint biomechanics. These studies illustrated the importance of lateral offset restoration during hip replacement surgery,³ a topic that continues to be of contemporary focus particular to contemporary stem design.⁴

Today, almost every orthopedic device manufacturer has at least one cemented Charnley-like stem within its product offerings, clearly indicating how this original design has been adapted to accommodate varying hip joint pathologies in the last three decades.

In the mid-1980s, the innovation of head/neck modularity emerged primarily to decrease the incidence of leg length inequality, minimize the risk of dislocation, and decrease component inventory. But with the introduction of these metal-metal interconnections, metallic debris and corrosion emerged as concerns to both the orthopedic surgeons using these devices and the scientists developing them.⁵ Both retrieval and laboratory investigations were conducted, determining that the risks associated with combining dissimilar metals (cobalt-chrome-molybdenum [Co-Cr-Mo] femoral heads, titanium alloy [Ti-6Al-4V] femoral stems) were outweighed by the biomechanical and biological benefits.^6-9

To further minimize acetabular component wear, contemporary modular femoral heads are manufactured from ceramics and have oxidized ceramic coatings. Femoral head/neck modularity has had clinical success for >20 years and remains an important element in the armamentarium of orthopedic surgeons.

While femoral head/neck modularity was becoming an industry-wide standard, total hip arthroplasty was battling the in vivo longevity limitations of polymethylmethacrylate (PMMA). Cement mantle fracture and aseptic loosening were significant issues in both primary and revision settings. Concurrent with improving cementing techniques (eg, vacuum mixing, pressurizing), cementless arthroplasty emerged. Several orthopedic device manufacturers developed coatings and/or surface treatments along with press-fit stem geometries to promote bone ingrowth and ongrowth to achieve component stability within the intramedullary canal.^10,11 Although a number of these systems have disappeared from the marketplace, they were responsible for determining optimized stem geometries, coating thickness, and surface treatment options. Today, 34% of all primary femoral stems implanted in the United States are cementless,¹² which clearly defines the clinical implementation of this innovation (Figure 1).

With the success of cementless arthroplasty technologies, modularity was developed as part of femoral stem component design.¹³ These devices offer an orthopedic surgeon a range of options to address the most complex hip pathologies. By mixing and matching proximal sections with distal stems and femoral heads, anteversion and lateral offset can be independently dialed in and optimized intraoperatively. However, the question of fretting at the multiple metal-metal interconnections was again raised because these locations can serve as stress risers for crack initiation and stem fracture.

Figure 2: A typical structural fatigue curve. The endurance limit defines the maximum dynamic load an implant system can support and, theoretically, never fails. The implant service load is defined as the maximum in vivo dynamic load on the hip during walking gait. The margin of safety is the difference between the endurance limit and the implant service load and serves as an indicator of implant structural integrity.

Scientists working with the International Standards Organization (ISO) had previously developed several testing protocols to evaluate the strength of distally supported, cemented monoblock femoral stems (ISO 7206) under different loading parameters (eg, in-plane and out-of-plane with subsequent torsion). However, these methods were not clinically relevant for proximally supported, cementless modular stem designs. Testing protocols were devised, which allowed for the development of a structural fatigue curve for these systems.¹⁴ A structural fatigue curve defines the maximum compressive load an implant system can sustain through 10 million uninterrupted cycles without device failure (endurance limit) (Figure 2). When this value is compared to the maximum implant service load realized through walking gait, a margin of safety is defined. The endurance limit is dependent on factors of patient weight, walking speed, and stride length. This methodology identifies the effects of changing geometries, materials, and manufacturing processes on specific implant longevity.

Extensive tests involved a structural fatigue curve for the S-ROM (DePuy Orthopaedics Inc., Warsaw, Ind), leading to the ability to evaluate any proximally supported modular femoral component by simulating anticipated bone ingrowth under fatigue loading.^15,16 The published results of these tests indicate that although fretting occurs in the modular designs that were evaluated and is an inevitable consequence of metal-metal interconnections, fretting did not significantly weaken the overall strength of the components. Although some designs have been discontinued from clinical use, the concept of modularity in femoral stem design is well accepted, as evidenced in the product line of almost every orthopedic device manufacturer.

Modular femoral stems are now designed to address complex revision situations of bone loss and deformity. This is accomplished through the offering of increased distal stem lengths, larger femoral head diameters, enlarged proximal segments, and curved stems, which are often used in combination with structural and morselized bone graft.^17-19 Contemporary modular designs offer both proximal and distal fixation options^20-22 and may be simply denominated in terms of their in vivo function: proximal fixation with distal stability, distal fixation with proximal stability, and proximal and/or distal fixation (Figure 3).


Figure 3: Contemporary modular stem designs typified by in vivo functional requirement: S-ROM (proximal fixation and distal stability) (A), Link MP Hip System (distal fixation and proximal stability) (Link Orthopaedics, Rockaway, NJ) (B), and ZMR Hip System-XL (distal fixation and proximal stability) (Zimmer Inc, Warsaw, Ind) (C).

Providing orthopedic surgeons with the ability to successfully treat patients presenting with hip pathology within a given hip system is an evolutionary approach to joint replacement technology. Through the combination of clinical insight and engineering intuitiveness, the modular femoral stem is only in its infancy and will continue to be optimized as orthopedic technologies expand.

References

Callaghan JJ, Templeton JE, Liu SS, et al. Results of Charnley total hip arthroplasty at a minimum of thirty years. A concise follow-up of a previous report. J Bone Joint Surg Am. 2004; 86:690-695.
2004 Hip and Knee Implant Review. Orthopedic Network News. 2004; 15:1.
Greenwald AS, Nelson CL. Biomechanics of the reconstructed hip. Orthop Clin North Am. 1973; 4:435-447.
Charles MN, Bourne RB, Davey R, Greenwald AS, Morrey BF, Rorabeck CH. Soft-tissue balancing of the hip. J Bone Joint Surg Am. 2004; 86:1078-1088.
Lombardi AV Jr, Mallory TH, Vaughn BK, Drouillard P. Aseptic loosening in total hip arthroplasty secondary to osteolysis induced by wear debris from titanium-alloy modular femoral heads. J Bone Joint Surg Am. 1989; 71:1337-1342.
Bobyn JD, Tanzer M, Krygier JJ, Dujovne AR, Brooks CE. Concerns with modularity in total hip arthroplasty. Clin Orthop. 1994; 298:27-36.
Collier JP, Surprenant BA, Jensen RE, Mayor MB. Corrosion at the interface of cobalt-alloy heads on titanium-alloy stems. Clin Orthop. 1991; 271:305-312.
Cook SD, Barrack RL, Baffes GC, et al. Wear and corrosion of modular interfaces in total hip replacements. Clin Orthop. 1994; 298:80-88.
Lieberman JR, Rimnac CM, Garvin KL, Klein RW, Salvati EA. An analysis of the head-taper interface in retrieved hip prostheses. Clin Orthop. 1994; 300:162-167.
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.
Engh CA, Hopper RH Jr. The odyssey of porous-coated fixation. J Arthroplasty. 2002; 17(4 Suppl 1):102-107.
2004 Bone Cement Update. Orthopedic Network News. 2004; 15:16.
Christie MJ, DeBoer DK, Trick LW, et al. Primary total hip arthroplasty with use of the modular S-ROM prosthesis. Four to seven-year clinical and radiographic results. J Bone Joint Surg Am. 1999; 81:1707-1716.
Polando G, Postak PD, Pugh JW, Greenwald AS. A new method of fatigue testing for proximally supported femoral stems. Paper presented at: Scientific Exhibit at the 57th Annual Meeting of the American Academy of Orthopaedic Surgeons; 1990; New Orleans, La.
Heim CS, Postak PD, Greenwald AS. Femoral stem fatigue characteristics of modular hip designs - Series II. Orthopaedic Transactions. 1995; 19:471-472.
Heim CS, Postak PD, Greenwald AS. Femoral stem fatigue characteristics of modular hip designs. In: Marlowe DE, Parr JE, Mayor MB, eds. Modularity of Orthopaedic Implants - STP 1301. Philadelphia, Pa: American Society for Testing and Materials; 1997:226-243.
Cameron HU. The long-term success of modular proximal fixation stems in revision total hip arthroplasty. J Arthroplasty. 2002; 17(4 Suppl 1):138-141.
Crawford CH, Malkani AL, Incavo SJ, Morris HB, Krupp RJ, Baker D. Femoral component revision using an extensively hydroxyapatite-coated stem. J Arthroplasty. 2004; 19:8-13.
McAuley JP, Engh CA Jr. Femoral fixation in the face of considerable bone loss: cylindrical and extensively coated femoral components. Clin Orthop. 2004; 429:215-221.
Goldberg VM. Revision total hip arthroplasty using a cementless modular femoral hip design. Am J Orthop. 2002; 31:202-204.
Kwong LM, Miller AJ, Lubinus P. A modular distal fixation option for proximal bone loss in revision total hip arthroplasty: a 2- to 6-year follow-up study. J Arthroplasty. 2003; 18(3 Suppl 1):94-97.
Murphy SB, Rodriguez J. Revision total hip arthroplasty with proximal bone loss. J Arthroplasty. 2004; 19(4 Suppl 1):115-119.

Authors

Ms Heim and Dr Greenwald are from the Orthopaedic Research Laboratories, Lutheran Hospital, Cleveland Clinic Health System, Cleveland, Ohio.