Introduction

Cementless tapered wedge stems have shown excellent results over the last decade. Distal potting with inadequate proximal fit, as well as failure to achieve biologic fixation has led to thigh pain, loosening and implant failure. To support a variety of patient morphologies a novel tapered wedge stem was designed with reduced distal morphology, maximizing the proximal contact of the grit blasted surface. The objective of the study was to analyze the clinical outcomes of this stem design.

Background

Instability and dislocation are some of the most important postoperative complications and potential causes of failure that dual mobility total hip arthroplasty (THA) systems continue to address. Studies have shown that increasing the relative head size provides patients implanted with smaller and larger cups increased stability, greater ROM and a lesser incidence of impingement, without compromising clinical results. The purpose of the current study was to review clinical outcomes in three groups of primary THA patients receiving a dual mobility acetabular shell.

Cementless biologic fixation surfaces on total joint replacement devices, such as those used in total hip and knee procedures, have evolved over the decades. Historically, various surfaces to allow bone ingrowth or ongrowth have been applied as a coating to a pre-formed solid metal substrate. As shown in Figure 1, from left to right, representative coating surfaces include sintered beads, diffusion-bonded fiber metal, and plasma sprayed titanium. In certain applications, tantalum porous metal (Fig 1, left) can be used without a solid metal substrate, but its most widespread usage is in a modular acetabular cup design with the porous metal diffusion-bonded to a solid metal substrate similar to other coatings. Each of these examples of biologic fixation surfaces has limitations. With comparatively low porosity, bead, fiber metal and plasma spray coatings are simply a surface enhancement onto a rigid machined, forged or cast metal substrate. Furthermore, the thermal process to apply the coatings can adversely affect the mechanical properties of the metal substrate. Released in the 1990's, tantalum porous metal is considered a ‘highly porous metal’ with twice the porosity of the applied surface coatings. With that greater porosity comes lower strength that requires engineers to make standalone tantalum porous metal shapes more bulky. The chemical deposition process to produce tantalum porous metal shapes has also limitations on geometry possibilities. Where bonding the tantalum porous metal to a solid metal substrate is necessary for adequate strength, that diffusion bonding process pressure can diminish the surface coefficient of friction necessary for initial stability.

A new class of manufacturing processing, referred to as ‘additive manufacturing’, allows engineers to create unique porous configurations. These configurations can be fabricated with beneficial properties to a specific implant application. One such enabling additive manufacturing process is called direct metal laser sintering (DMLS). This process utilizes a laser that travels over a fine powder bed. The laser path is determined by a program that mimics a computer model. Where the laser contacts the powder bed, the powder consolidates. Layer by layer, a scaffold porous metal is fabricated. Figure 2 shows a titanium alloy porous metal structure produced by DMLS. This formed biomaterial has 65% porosity, a high coefficient of friction, low stiffness, and strength that is 2 to 3 times that of tantalum porous metal. From a design versatility perspective, with greater strength, relatively thinner and more bone conserving geometries can be developed. When a solid metal surface interface to secure a modular polymer bearing is required, the DMLS process can produce the solid surface and the porous metal at the same time. With no secondary bonding thermal cycle needed, the construct's mechanical integrity is not compromised. Advancing biologic fixation necessitates bone conserving implant designs that have the properties to achieve immediate mechanical stability and longer term bone ingrowth. This novel use of DMLS in this particular porous metal geometry allows engineers to meet those criteria.

Various reports confirm that elevations in serum markers associated with skeletal muscle injury exist and can occur after orthopaedic surgery in the absence of overt clinical manifestations of myocardial injury. The purpose of this study is to measure the influence surgical approach on these serum markers following primary Minimally Invasive THA. Consecutive enrollment of 30 patients into three different groups of 10 was performed. The MIS Modified Watson Jones THA is an approach using an inter-muscular plane, the Mini Posterior is a trans-muscular approach with some muscle detachment and repair, while the MIS II Incision THA is an inter-muscular approach anteriorly and a trans-muscular approach posteriorly. Blood samples for total creatine kinase (CK), creatine phospho-kinase (CPK), and serum myoglobin were obtained at screening and the morning before surgery as a baseline, immediately post-operatively in the recovery room and 8, 16, 24, 36, 48, and 72 hours post-operatively. Hemoglobin and hematocrit was obtained pre-operatively, 16, 36, and 72 hours (±6 hours) post-operatively. Cardiac troponin-I was measured the morning before surgery (pre-operatively) and 16 hours following surgery to monitor any contributory effect of myocardial injury. We report measurable and reproducible trends in serum enzyme levels consistent with skeletal muscle damage due to THA. Troponin-I remained normal in all but one case throughout the entire study indicating no myocardial contribution to measured serum enzyme levels. While these trends may have slight correlation with surgical approach, they were not statistically significant. We conclude that all three procedures do affect serum enzyme markers and are safe from this standpoint, but no surgical approach appears to affect the degree of muscle trauma more or less than another.

The purpose of this study was to compare the clinical, radiographic, and DEXA results of Epoch® Femoral Component for primary THA with other non-cemented femoral components. The Epoch and Epoch 14+ (Zimmer, Warsaw, IN) Composite Femoral Components were studied in conjunction with the VerSys® Fiber Metal Taper, Fiber Metal Mid coat and Beaded Fullcoat Femoral Components (Zimmer, Warsaw, IN). All patients were randomized into one of five component groups and followed prospectively. All patients across the five groups were matched with respect to demographics and body mass index (BMI) (Table 1). Diagnoses were evenly distributed across all study groups. There was no significant change in BMD at 5-years when compared to the baseline (6 month) DEXA measurements for all component groups except for the Beaded Fullcoat component subset which had significant decreases in BMD in proximal zones 1, 6 and 7, The Fiber Metal Taper showed similar decreases in BMD in zones 1 and 7, and the Epoch Component had a significant decrease in BMD in zone 7 only. Additionally, our DEXA findings complement and support the subjective radiographic interpretations performed for this study cohort. The results from this prospective, randomized controlled clinical trial showed that the Epoch Composite Femoral Component for primary THA can achieve results equal to standard alloy components of varying design. Based on these early results, the Epoch and Epoch 14+ Femoral Components for primary THA are justified for continuance of further prospective study.

Modularity of femoral components has been widely accepted at the head neck junction, most commonly combining two unlike metals with only sporadic reporting of compatibility issues and corrosion. The development and introduction of a new and improved modular neck junction (Rejuvenate Modular Femoral component, Stryker Orthopedics) provided the option of fine-tuning leg lengths, offset and stability. The surgical technique did indeed provide the desired endpoints, however, the early recognition of problems with the junction causing corrosion and Adverse Local Soft Tissue Reaction (ALTR) and subsequent revision has led to the product being voluntarily withdrawn from the market. My experience as an early user of this stem is described in this manuscript providing a better early recognition and treatment of this potentially very destructive process.