Proximal Porous-Coated Modular Stems: Surgical Technique

CASE STUDY

A 63-year old male patient presented with chief complaints of persistent thigh pain and restriction of function at 1 year following primary total hip arthroplasty (THA) performed for degenerative hip arthritis. A proximally porous coated tapered wedge stem was used during primary surgery.

Continuous radiolucencies were seen in Gruen zones 1, 2, 6, and 7 on anteroposterior radiographs ( Fig. 62.1, A ) and zones 8 and 9 on lateral radiographs ( Fig. 62.2, B ). A pedestal was seen near the distal lateral cortex at the tip. Thus, radiographs were suggestive of failure of osseous integration of the primary stem.

Patient underwent revision THA due to persistent functional restriction and thigh pain. Explantation of the femoral implant resulted in a type I Paprosky bone loss, making it difficult for a larger primary-tapered wedge stem to obtain stable fixation in the meatphysis. Emperion stem was used to achieve adequate fit and fill in the meatphyseal and diaphyseal regions. Restoration of offset, leg-length, and stable fixation were achieved with the use of an Emperion stem (see Fig. 62.2 ). Patient reported subsequent resolution of symptoms and improvement of functional activity. The principles of revision surgery with this type of stem are discussed here.

Introduction

In concept, a modular femoral prosthesis allows the use of an independent proximal sleeve to achieve axial fixation and osteointegration while a distal stem serves to customize reconstruction of the center of rotation according to the anatomy of the femur at primary or revision surgery. Modularity also enables placement of implant version irrespective of fixation to maximize the range of motion and improve stability. High-offset options can help achieve optimum biomechanics without negatively affecting leg length.

This chapter reviews the surgical technique of revision total hip arthroplasty (THA) performed with the use of a proximal porous-coated modular stem (Emperion; Smith & Nephew, Memphis, Tenn.). The principles of revision surgery are common for both of the prototype stems in this group: Emperion and S-ROM (DePuy Orthopaedics, Warsaw, Ind.).

Equipment

Stem Options

The principal features of the Emperion stem design include a polished cylindrical distal geometry, the circulotrapezoidal geometry of the neck, and the presence of flutes and a coronal slot in the distal part of the stem with a polished bullet tip. The circulotrapezoidal neck is designed to provide a greater arc of motion before neck impingement. Distal flutes improve rotational stability and are important for immediate fixation of the stem in order to allow for osteointegration. The presence of a coronal slot decreases stiffness of the stem and makes its insertion easier. The bullet tip aims to decrease stem–bone impingement and decreases stress transfer at the stem tip.

The instrumentation is designed so that the surgeon can implant a stem the same size as the last distal reamer used. For example, a 14-mm stem is implanted if a 14-mm reamer was the last one used. To achieve press-fit, each stem flute dimension is oversized by 0.5 mm compared with the final reamer size. The entire technique and procedure is then based on the distal stem diameter.

Stems are available in sizes ranging from 9 to 23 mm in 2-mm increments. Three stem lengths are available: primary, standard revision, and long revision (which are bowed). The revision stems are available only with high-offset options. They have a calcar replacement option as well (+10 calcar replacement for standard revision; +10 and +20 calcar replacement options for long revision stems). All stems have uniform neck length.

S-ROM stems are available in sizes ranging from 11 to 19 mm in 2-mm increments. There are three standard neck lengths and four lateral offset options; this helps to optimize soft tissue tension without changing the head selection.

Sleeve Sizes

The sleeves of the Emperion system ( Fig. 62.3 ) are porous coated with sintered bead technology and have a pore size of about 200 μm. A 50-μm-thick layer of hydroxyapatite (HA) coating is applied to this porous coating. The proximal sleeve design incorporates a 6-degree taper (3 degrees on each side). This taper improves proximal fill and enhances loading of the proximal femoral bone. In addition, the actual implant is oversized by 0.5 mm compared to the area machined by the instruments, which provides for an interference fit and enhanced initial fixation. The interference fit and tapered sleeve geometry are designed to transfer stress to the host bone and minimize bone loss from stress shielding.

The Emperion system has two sleeve length options available: 40 mm (standard) and 60 mm (tall). The tall sleeve is useful for revision THAs in which the amount of bone loss necessitates the extension of fixation more distally into the diaphysis.

The nomenclature of sleeves is based on the diameters of the “cone portion” and the “spout portion.” The proximal sleeve name carries the stem size with which it is associated (e.g., an 11-mm sleeve is used with an 11-mm stem). The cones come in sizes of small, medium, and large, and extra-large. There is a 2-mm difference between cone sizes. The spouts come in sizes of 1, 2, and 3, with a 4-mm incremental difference between spout sizes. For stem sizes 15 mm and smaller, a total of nine different standard sleeve options are available; the fourth cone size (extra large) is available only for stem sizes 17 mm and larger. On the other hand, the tall sleeves have two cone sizes (small and medium) and two spout sizes (1 and 2), making a total of four different tall sleeve options for each revision stem size.

In the S-ROM system, three cone sizes (B, D, and F) are available for each stem size; they add 3 mm to the proximal diameter of the stem in 2-mm increments. In addition, three spout sizes are available for each cone size (extending about 9.5 mm from the cone in 4-mm increments). To achieve accurate fit in the metaphysis, each standard stem matches up to ten proximal sleeves with varying diameters and spout sizes. Sleeves are available with ZTT porous coating and ZT HA coating. The stepped design converts shear forces to compressive forces to achieve proximal stability and prevent subsidence in a well-fixed sleeve ( Fig. 62.4 ).

Surgical Technique

Preoperative Planning ( Box 62.1 )

Thorough preparation incorporates information gathered from the history, clinical examination, and radiographs. In the revision setting, it is important to give consideration to the following:

1.

The cause of revision. Is infection present?
2.

Loosening of implants and associated bone loss. Type IIIB and type IV Paprosky bone loss are associated with poor outcome with this type of stem.
3.

Cemented THA. If present, removal of cement and special equipment are required.
4.

Uncemented THA. If present, planning for osteotomy (extended trochanteric) may be required for the removal of well-fixed components. Modular stems with proximal sleeves are usually not preferred if an extended trochanteric osteotomy is required.