Acetabular Revision with Metal Shell Retention

Three additional studies have examined constrained polyethylene liners that were cemented into well-fixed shells. Due to the increased shear forces that may be generated by constrained cups at the cement interface, these series likely involve the worst-case scenario for cemented liners. Perez et al.⁷ reviewed 10 patients who received cemented constrained PE liners into well-fixed acetabular components for the treatment of recurrent dislocation. At a mean follow-up of 2 years (range 1 to 3.2 years), the authors reported that nine (90%) of the hips continued in service without evidence of loosening. Shapiro et al.²⁸ looked at a series of constrained acetabular components, 16 of which were cemented into well-fixed acetabular shells, and found that 14 of 16 (87.5%) were still functioning at a mean follow-up of 4.8 years. Callaghan et al.²⁹ reviewed 31 liners that were cemented into well-fixed cementless shells, and found only one liner that had loosened from the cement mantle at a mean follow-up of 3.9 years.

These studies have demonstrated the importance of proper patient selection, accurate sizing of the PE liner, careful preparation of the substrate of the liner and the shell, and good cement technique. The potential advantages of this technique are less surgical morbidity, more rapid surgery and patient recovery, the ability to incorporate antibiotics into the cement, and more liner options. This chapter reviews the steps for successful liner implantation.

DECIDE IF RETAINING THE SHELL IS AN OPTION

When liner cementation is considered, the stability of the existing shell should be assessed critically by reviewing serial radiographs of the hip. The presence of continuous radiolucent lines, migration, or large osteolytic lesions may indicate impending loosening and preclude the retention of the shell. The surgeon should be alert for a history of activity-related groin pain that could suggest impending loosening of the acetabular shell. The previous operative note and hospital record should be reviewed to learn about the manufacturer of the existing shell, the diameter and thickness of the shell (which gives information about the inner shell diameter), and the geometry of the inner surface of the shell to ensure that the proper liner is available in the operating room. The identity of the old component is useful to determine whether a replacement polyethylene is available. In terms of fixation strength, Bonner et al.²³ examined the variables of liner diameter (compared to shell diameter), type of liner (all PE vs. modular design), and liner modification (grooves and their orientation) and found that the relative size of the liner to the inside diameter of the shell was by far the most important variable. If a surgeon contemplates using a liner from a different manufacturer than the shell manufacturer, every attempt preoperatively should be made to get a copy of the acetabular component and compare it to the intended liner to ensure that the geometry of the liner will be compatible with the new shell.

Equipment to be assembled preoperatively includes the standard equipment required for any acetabular revision. The surgeon should also obtain the insertion handle for the original cementless shell in order to more properly test the stability of the shell. In addition, that manufacturer may make an extraction device that will assist in the removal of the liner. Also available should be a 6.5-mm cancellous screw set to assist in difficult liner removal (see below), high-speed burr (including carbide bits) if it is necessary to make grooves in the cup or liner, and new sockets in case the socket is loose. Even if significant osteolytic lesions are not seen on preoperative radiographs, particulate bone graft should also be available should smaller lesions be found around the cup. These lesions can sometimes be accessed through dome screw holes in the metal shell, or through trapdoor approaches above the shell, as outlined by Maloney et al.³⁰

REMOVE THE EXISTING LINER

Most first-generation liners can be removed with osteotomes.¹² Most manufacturers make extraction devices for newer sockets which have liners that can be difficult to remove (without that tool), particularly when there is a metal ring locking type of mechanism. If there is no extraction device available, the surgeon can drill into the polyethylene with a 4.5-mm drill (halfway between the apex of the cup and the rim), insert a 6.5-mm screw, and as the screw tip contacts the acetabular shell, the polyethylene liner is lifted away from the shell. If this is unsuccessful, the surgeon can divide the polyethylene into quarters using a high-speed pencil tip burr or osteotome. The rim of the liner should be inspected carefully before removal for signs of impingement of the femoral neck against the polyethylene rim that may indicate unacceptable socket position, need to change the position of an elevated liner rim, or need to improve the head-to-neck ratio (usually by using a large head size).

TEST THE EXISTING METAL SHELL

With the liner removed, the surgeon should expose the metalbone interface of the shell and test the cup stability. The techniques of grasping the cup rim with a Kocher clamp or pushing on the rim with a punch may not provide sufficient force to demonstrate lack of bone ingrowth. The surgeon should make every effort to have available the insertion handle specifically made for the socket in question as this will allow a better test of the interface stability. If this is unavailable, many manufacturers produce tools that will grab the rim of the socket, as well as devices that can lock into screw holes in the shell. Motion demonstrated at the interface should trigger revision of the shell.

The surgeon should assess the surface finish of the inside of the shell as well as the internal topography of the socket. There are several situations where the macrointerlock of the new liner against the existing shell is less than optimal. For example, Kumar et al.,²⁶ in their biomechanical study, noted that metal shells with five dome screw holes exhibited 40% greater fixation strength of a cemented liner than shells without holes. They also noted that a polished shell interior decreases the fixation strength. If there is a small rim on the metal socket, this will decrease the macrointerlock of the shoulder of the liner against the metal rim, and fixation strengths will be decreased. If a constrained liner is planned, this will generate additional stresses at the liner-cement and cement-shell interfaces. In these settings, the surgeon should consider creating grooves on the inner surface of the shell with metal cutting instruments to improve the interlock.

SELECT AND SIZE THE NEW LINER

The surgeon should consider what types of liners are needed for the particular clinical application so that these types are available in the operating room. This includes not only the range of liner sizes for the shell in question but also the material, the inside diameter, and different liner face options. If the risk of dislocation is high, the surgeon may have constrained liners, or liners compatible with extra-large head diameters or bipolar heads available. Constrained liners should be avoided if there is concern for impingement of the femoral neck against the elevated lip of the liner within the normal range of hip motion, as repeated impingement can threaten the fixation of the cemented liner in the shell, the shell in the bony interface, and the constraining mechanism. This being said, series of constrained acetabular liners cemented into existing metal shells have not shown increased loosening rates over unconstrained liners cemented into existing shells.^29,31

A full complement of trial liners for the intended liner should be available in the operating room. The PE liner does not necessarily need to be manufactured by the same company as the metal shell, but the surgeon should be alert for differences in the backside geometry, which could lead to incompatibility with the shell. There are substantial differences between PE liners in terms of the shoulder geometry and rim size (Fig. 10-1). Some metal shells have tabs on the rim that can interfere with seating of the liner (Fig. 10-2). Not all liners are designated by their true outside diameter (Fig. 10-3). In addition, the thickness of the metal shell varies between manufacturers, and thus the inner diameter of the shell may be difficult to estimate, which highlights the need to communicate with a representative from the company that made the shell. Several authors have shown that a liner that is undersized 2 to 4 mm versus the inside diameter of the shell will give superior fixation strengths than liners that are 2 mm larger than the inside diameter of the shell.^22,23,25 Bonner et al.²³ showed that PCA liners (Howmedica, Rutherford, NJ) with diameters 2 mm greater than the metal shell inner diameter failed at lever-out loads of 89.67 ± 1.53 N, whereas those same liners when undersized 2 mm failed at loads of 454.25 ± 44.09 N. The conventional locking mechanism control failed at a lever-out load of 176 ± 23 N. Meldrum and Hollis²⁵ found that the lever-out strength of an undersized (4 mm) cemented PE liner in an HGP (Zimmer, Warsaw, IN) shell was 150 ± 40 lb (672 N). Heiner et al.¹²