CHAPTER 22 The Cementless Modular Stem

Encouraging results have been shown in the evolution of cementless femoral implants for total hip arthroplasty. Some early designs resulted in unacceptable rates of aseptic loosening that led to a focus on improved initial stability by optimizing femoral fit and fill. Anatomic, straight, and even custom stems were developed in attempts to improve fixation. This cementless femoral evolution has resulted in modular design concepts that not only improve initial stability by improved “fit and fill” but also address the highly variable femoral geometries that monolithic stems cannot address.

Modularity in orthopedic implant designs is not a new concept—it has been used for years in total hip arthroplasty in both acetabular and femoral components. The basic premise is that the modular connections in these devices allow the surgeon to customize the implant geometry to better match patient needs and anatomy.

For more than 20 years, modularity has been proven successful in acetabular shell designs, allowing for the selection of the bearing type for the acetabular liner (e.g., metal on metal, metal on polyethylene; ceramic on polyethylene, ceramic on ceramic), offset (eccentric liners), and even geometry (hooded, elevated rim, constrained liners), providing a range of choices for surgeons to match individual patient needs both preoperatively and intraoperatively. Modularity also provides an option for revision of the liner in revision procedures. These choices are critical in addressing the key needs of the implant to restore function, alleviate pain, prevent dislocation, and enhance range of motion.

In the femoral stem, modularity has allowed for customization of the femoral head, and modern-day devices have seen this concept of modularity applied in new ways to address issues of anteversion control, neck angle, leg length, lateral offset, proximal and distal sizing, and even varying geometries and surface coatings. Customized, cementless, computer-assisted design, and manufactured femoral components as advocated by Bargar and colleagues¹ are seldom needed with the intraoperative flexibility that modular stems provide.

For total hip arthroplasty there are several types of femoral stem modularity, with two distinct types being the most prevalent: midstem modularity and proximal modularity. Examples of midstem modularity include the ZMR hip stem (Zimmer, Warsaw, IN), Mallory-Head (Biomet, Warsaw, IN), Restoration Modular (Stryker, Kalamazoo, MI), and Link MP (Link Orthopedics, Pine Brook, NJ). Proximal modularity designs include the S-ROM (DePuy, Warsaw, IN) and the ProFemur (Wright Medical Technology, Arlington, TN).

In primary total hip arthroplasty the prerequisites for long-term implant survival and performance are well documented. The implant must achieve initial fixation and long-term stability. It must also re-create hip joint mechanics. Because all patients have an inherently different femoral anatomy and can present with abnormalities at the time of surgery, modular stems allow the surgeon to customize the implant design to address these needs and achieve the desired goals.

Although extensively coated stems for primary hip applications have shown encouraging results, most primary cementless implants focus on proximal fit and ingrowth for long-term stability. The importance of combining proximal fit with appropriate distal fit to control micromotion, facilitating improved initial implant stability and potential for ingrowth, has been shown by Whiteside and Easley.² Modular stems allow the surgeon to maximize the proximal and distal fit of the implant.

In many cases, the need for modular stems to better address natural anatomy can be determined through preoperative evaluation of radiographs. However, these types of stems are also a valuable backup should a traditional primary hip stem fail to adequately address the femoral preparation. Midstem modular implants can address proximal-distal sizing mismatch in the femoral canal. The success of these procedures relies on the surgeon’s ability to ensure an intimate fit of the implant with the host bone and provide structural support for the device. Midstem and proximally modular systems can also address biomechanical issues of neck length or offset, neck angle, femoral anteversion, and leg length. The use of “skirted” femoral heads can increase contact stresses at the taper interface and result in impingement on the acetabular component, with concerns of wear, instability, and decreased range of motion. With the use of modular stems, “skirted” femoral heads should be virtually eliminated. Furthermore, modular stems can provide a construct that will load the femur proximally, distally, or a combination thereof, with the development of extensively porous coated and tapered stem geometries. Load transfer to the femur can help maintain bone mass as more physiologic stresses are applied.

Modular systems also facilitate greater accuracy, precision, and reliability in femoral canal preparation. The instrumentation used in preparation for modular stems gives the surgeon the capability of preparing the distal femur independent of the proximal femur—and may even aid in the insertion of the device, especially when negotiating the anterior bow of the femur—without compromising proximal anteversion relative to the distal bow. This is an area that is a challenge with monolithic implants that have fixed degrees of anteversion. Furthermore, modular stems can treat inadequacies in the combined femoral-acetabular anteversion by adjusting the proximal component.

For proximal femoral deformity addressed with corrective femoral osteotomies in the setting of a primary hip replacement, modular implants provide a firm foundation for achieving stability in the canal and then building up over a well-fixed stem, to cater to hip joint mechanics in a manner that monolithic stems cannot.

Modular stem designs have afforded some surgeons the ability to prepare the femur through smaller incisions and various approaches, without disrupting the natural anatomy as in traditional total hip arthroplasty. For example, the neck on a proximally modular stem can be positioned after the femoral component has been implanted in the canal, reducing the need to excise excessive amounts of bone or soft tissue. However, surgeons need to be cognizant of the learning curve associated with these techniques and with the use of modular implants and instrumentation. Surgeons should take great care in modifying their techniques, stem choices, or approaches and limit their modifications to one controllable variable.

Modular stem designs may also enhance the surgeon’s ability to remove or reposition the component. Removal of proximal configurations in the revision of the stem gives the surgeon the ability to discretely visualize the implanted components, either in the distal femur or in the acetabular component (e.g., with a liner exchange). Isolated removal of the proximal body or neck reduces the need for a total femoral revision by simply replacing the modular component to achieve the desired goal.

Modular junctions are not without their drawbacks; namely, their comparative weakness. There have been several instances of breakages of the stem at the taper junction, although newer designs have alleviated some of these concerns. Advances in manufacturing techniques allow us to push the limits of the materials’ properties while ensuring that the true benefits that the implant concept provides are retained. However, surgeons must consider the placement and design of the taper junction, because the taper in the midstem modular system is subject to varying forces that do not affect the proximal modular implant in the same way and therefore the taper must be supported by good quality host bone.

Although there are many designs for modularity in femoral stems, surgeons should be aware of their features, benefits, and applications. Surgeons should also know the limitations of these designs and how they can affect the patient being treated. Modular systems are limited in their ability to replicate the anatomy of severely small femoral anatomies achieve the relative strength of traditional monolithic implants. Modular systems also increase the number of options, making preoperative templating critically important to effective implant selection.

Related posts:

The Cementless Tapered Stem Femoroacetabular Osteoplasty Reconstruction of Acetabular Bone Deficiencies Using the Antiprotrusio Cage The Cemented Stem Revision Total Hip Replacement The Cemented Stem

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