Evolution of Stem Design and Direct Anterior Approach-Specific Technologies

Robert A. Sershon

Eric M. Cohen

Tyler D. Goldberg

William G. Hamilton

Introduction

The direct anterior approach (DAA) for total hip arthroplasty (THA) is safe and effective.¹ The advantages of the DAA include preservation of soft tissue, more accurate acetabular component positioning, improved restoration of leg lengths, low dislocation rates, earlier recovery, and shorter hospital stay.¹^,²^,³^,⁴ The disadvantages of the DAA often relate to the femur, with authors citing greater difficulty in femoral exposure and preparation leading to a potential increased risk of mechanical complications including periprosthetic fracture and aseptic loosening.¹^,⁵ To address femoral exposure challenges unique to DAA, advancements in implant design, instrumentation, and techniques to enhance the DAA procedure continue to evolve. In this chapter, we discuss the past, present, and future of instrumentation and implants as they relate to the DAA approach, with a heavy emphasis on the femoral side.

Stem Classification

Multiple stem classifications have been proposed, with no universally accepted system.⁶^,⁷^,⁸ Khanuja et al⁷ proposed a femoral stem classification system with six different stem types based on shape (Table 53.1). Because of the diversity of the available stems, the classification scheme overlooks some of the most commonly used hydroxyapatite (HA)-coated compaction broaching designs (Table 53.2). Attempts at classifying newer, short-stem, and bone- conserving stems have also been made,⁸ although robust clinical evidence for some of the described implants is lacking.

TABLE 53.1 Traditional Cementless Stem Design Classification⁷

Type	Geometry	Coating	Fixation	Examples
1	Single-plane wedge taper	Proximal ± distal coating or roughening	Metaphysis	Tri-Lock, Linear, Alteon, Anthology, Accolade II, Taperloc, M-L Taper
2	Dual-plane wedge taper, metaphyseal fill	Proximal ± distal coating or roughening	Metaphysis Metadiaphysis	Summit, Novation, Synergy, Secur-Fit, Bi-Metric, Trabecular Metal Primary
3A	Tapered, round	Proximal two-thirds coated	Metadiaphysis Metadiaphysis	Mallory-Head, PCA, Harris-Galante
3B	Splined, tapered conical	Grit-blasted Ti	Metadiaphysis Diaphysis	Wagner, Redapt
3C	Rectangular, tapered	Grit-blasted Ti	Metaphysis Metadiaphysis	Zweymuller, Fitmore
4	Cylindrical, collared	Fully coated CoCr	Metadiaphysis Diaphysis	AML, VerSys, Echelon
5	Modular	Variable	Metaphysis Metadiaphysis	S-ROM, Arcos, Reclaim, Restoration
6	Anatomic	Variable	Metaphysis	PCA
Adapted with permission from Khanuja HS, Vakil JJ, Goddard MS, Mont MA. Cementless femoral fixation in total hip arthroplasty. J Bone Joint Surg Am. 2011;93(5):500-509. Copyright © 2011 by The Journal of Bone and Joint Surgery, Incorporated.

TABLE 53.2 Hydroxyapatite-Coated, Trapezoidal Compaction Broaching Stems

Corail (DePuy Synthes)

Exactech Novation Element (Exactech, Gainesville, FL, USA)

Quadra H (Medacta)

Polarstem (Smith & Nephew, Memphis, TN, USA)

Avenir (Zimmer Biomet)

Since 2014, several short-stemmed, proximal “fit-and-fill” femoral designs have emerged to facilitate femoral preparation and implantation through the DAA approach (Table 53.3). These newer designs resemble type 2 designed implants (dual taper and metaphyseal filling) but have several unique design features described later in this chapter, such as triple-taper geometries and reduced lateral shoulders.

TABLE 53.3 Fit-and-Fill and Short Triple-Taper Designs Since 2014

ACTIS (DePuy Synthes)

TaperFill (DJO-ENOVIS)

AMIStem (Medacta)

Avenir Complete (Zimmer Biomet)

Echo Microplasty (Zimmer Biomet)

Evolution of Direct Anterior Approach Stem Use

Surgeons using the DAA found the use of straight reamers for canal preparation difficult because of the need for extensive anterior mobilization of the femur to allow canal access.⁹ In contrast with the posterior approach, a colinear broaching trajectory is more challenging to attain with the DAA. Conversely, colinear broaching via the posterior approach can require removal of trochanteric bone and can be associated with abductor tendon damage in patients with larger abductor musculature. In the anterior approach, the abductors fall posterior to the field protecting them from damage even when trochanteric bone removal is necessary.

The principal limitation in the DAA is difficulty in elevating the femur so that straight reamers and broaches can clear the skin. Femoral exposure can be challenging during DAA THA due to contracted capsular ligaments and tendons, hip morphology (short varus and long valgus), and patient body habitus. A multicenter study including 5090 consecutive anterior total hip replacements reported only 5.5% of implants being ream and broach systems.¹ The remaining stems in this series consisted of broach-only systems with reduced lateral shoulder stems and offset broach options. Through the use of such implants, less soft release is required for gaining adequate exposure, theoretically enhancing hip stability and reducing postoperative pain.

The use of flat tapered wedge titanium stems for DAA THA has remained a popular choice for two decades. These stems are typically low profile and broach only, possess reduced lateral shoulders, and have an excellent track record. Although there have been reports of higher fracture rates with longer type 1 stem use, recent investigations have demonstrated low rates of fracture with short type 1 stems.¹⁰^,¹¹^,¹²

Longer, trapezoidal compaction broaching designs with HA coating have also demonstrated excellent success with low revision rates.¹^,¹¹ These titanium stems use compaction broaches to pack instead of remove cancellous bone. They are designed with reduced lateral shoulders to decrease the exposure required for stem insertion, a trapezoidal cross section to promote initial stability, and tapered distal geometry to reduce distal cortical contact. Many designs have collared options, which have been shown from data in the National Joint Registry of the United Kingdom to enhance initial stability and reduce early fracture rates.¹³

Recent implant designs have aimed to retain a proximal fit-and-fill geometry with reduced lateral shoulders while shortening stem length (Table 53.3). Given their recent release, midterm results were yet unavailable at the time of this chapter’s writing.

Design Features Important in Direct Anterior Approach Total Hip Arthroplasty

Femoral Implant

The goal of any THA stem is to achieve primary stability, which allows for secondary osteointegration and durable fixation. Concerns over stem fixation and survivorship when using the anterior approach have been reported. Meneghini et al¹⁴ retrospectively reviewed 342 patients from multiple centers and found stems implanted using the DAA were more likely to require revision for femoral aseptic loosening compared with a posterior approach. Janssen et al¹⁵ reviewed 63,354 THAs in the Dutch Arthroplasty Registry with a minimum 2-year follow-up and found that stems with a proximal shoulder had a higher incidence of aseptic loosening compared with “anatomic,” reduced shoulder implants. Given reports such as these, surgeons and industry have sought to develop more anterior-friendly stem designs with the goal of decreasing complications and improving long-term survivorship.

Proximally coated tapered wedge femoral stems have a long track record of durable performance.¹^,⁶^,⁷^,¹⁰ Combined with a reduced lateral shoulder and impaction broaching, these features have made tapered wedge stems a natural solution to femoral preparation challenges in DAA THA. However, some authors have found increased rates of early subsidence, fracture, and loosening with a resultant increase
in revision rates.¹⁰^,¹¹^,¹³ Risk factors have included male sex, larger stem size, smaller canal fill index, and Dorr A bone.¹⁶ Furthermore, flat wedged implants are narrow in the anterior-posterior dimension. Rotation of the broach caused by a challenging exposure or poor broaching technique may result in metaphyseal gaps between the bone and the implant and diminish the initial stability. Furthermore, implanting a stem designed for proximal fixation into Dorr type A bone may cause potting of the distal aspect of the stem and failure of proximal fixation and can lead to loosening. This can be mitigated by reaming the diaphysis with flexible reamers to the diameter of the distal implant, preventing distal potting and allowing for appropriate proximal fixation. Consequently, proximally filling femoral stem geometries have been developed for DAA THA.

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