Early cementless acetabular components (first generation) had problems with locking mechanisms, liner designs with relatively thin areas of the PE, 2 and poor PE durability. 2,5,7,12,13,20,21,24 This has led to failure of the PE liner at intermediate periods, through either mechanical instability of the liner, wear, or both; although the metal socket and stem remained well-fixed. 26 There are numerous reports of dislocation of a modular PE liner because of failure of the locking mechanism, 6,8,10,14,17,18,22,27 and there was a report of dislocation of a nonmodular acetabular liner. 23 Some reports recommended revision of the entire acetabular component when the locking mechanism was damaged. Revision of an acetabular shell is not without risks, however, including fracture and significant bone loss. 19,25 This has led to studies of the feasibility of cementation of PE liners when a conventional liner cannot be fixed to the metal shell. 1,3,4,9,16 The common indications for cementing a PE liner include dislocation of a PE liner, a damaged locking mechanism (by wear, fatigue, or trauma), lack of a replacement liner of the desired material or size, the need for a face changing liner, or the need for a constrained liner.
Two clinical series of PE liners cemented into metal shells have been published. LaPorte et al 12 reported a series of eight hips in eight patients with the longest followup of 7 years (no mean followup or range was provided) and successful outcomes in seven hips (87%). Haft et al 9 reported 17 cemented PE liners in 17 patients with a followup of 2.5 years (range, 1–4.7 years). They reported that 16 hips (94%) remained in service without evidence of loosening at followup, and their one failure occurred at the PE-cement interface in a liner that was oversized.
Therefore, the cementing of a PE liner into a well-fixed cementless acetabular shell seems to be a viable alternative in selected cases, at short-term followup. I will review the key technical points to achieve cemented fixation of a PE liner into an existing metal shell based on available mechanical and clinical data.
MATERIALS AND METHODS
The stability of the existing shell should be assessed critically by reviewing serial radiographs of the hip. The presence of continuous radiolucent lines or significant 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 obtained, because it is important to know the manufacturer of the existing shell, and the diameter and thickness of the shell (the inner diameter), and the geometry of the inner surface of the shell to ensure that the proper liner is available in the operating room. In terms of fixation strength, Bonner et al 4 examined the variables of liner size, type of liner (all PE versus modular design), and liner modification (grooves and their orientation) and reported that size was by far the most important variable.
Removal of the Existing Liner
The identity of the old component is useful to determine whether a replacement PE is available. In addition, that manufacturer may make an extraction device that will assist in the removal of the liner. Most first generation liners can be removed with osteotomes. 11 If a newer generation liner must be removed, and there is no extraction device available, the surgeon can drill into the PE with a 4.5-mm drill, insert a 6.5-mm screw, and as the screw tip contacts the acetabular shell, the liner is lifted away from the shell.
Evaluation of the Existing Shell
With the liner removed, the surgeon should expose the metal-bone interface of the shell and test the stability. The technique of grasping the rim with a Kocher clamp or pushing on the rim with a punch may not provide sufficient force to show instability. The surgeon should make every effort to have the insertion handle made for the socket in question because this will allow an adequate test of the interface stability. If this is unavailable, many manufacturers produce tools that will grab the rim of the socket, and devices that can lock into screw holes in the shell. Motion shown at the interface should trigger revision of the shell.
Once the surgeon has verified adequate cup stability, all debris should be cleaned from the shell. If screws are present and are grossly loose they should be removed because they are not adding to stability and remain a conduit for particulate debris. Osteolytic lesions should be treated with particulate bone graft through the screw holes or by a trapdoor approach outside the shell. 4,15
Selecting the New Liner
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 source as the metal shell, but the surgeon should be alert for differences in the backside geometry that could lead to incompatibility with the shell. There are substantial differences between PE liners in terms of the shoulder geometry and rim size (Fig 1). Some metal shells have tabs on the rim that can interfere with seating of the liner (Fig 2). Not all liners are designated by their true outside diameter (Fig 3). In addition, the thickness of the metal shell varies between manufacturers, and therefore 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. 3,4,16 Bonner et al 4 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 Hollis 17 reported that the lever out strength of an undersized (4-mm) cemented PE liner in an HGP (Zimmer, Warsaw, IN) shell was 150 ± 40 lbs. (672 N). Heiner et al 11 reported similar lever out strengths in their study of Depuy shells and liners (Depuy, Warsaw, IN), but also showed that a 2-mm cement mantle was almost twice as strong as a 4-mm cement mantle in lever out tests.
Therefore, it seems that containing the liner within the shell is a critical element for obtaining good results in PE liners cemented into shells. Containment is important for two reasons: to prevent impingement of the neck of the stem against the liner, and to improve the mechanical contact of the liner rim against the face of the metal shell. Figure 4 shows the trial process to choose the proper liner. It is not clear from the literature whether cementable all-PE cups or PE liners are the best choice for cementing into a shell. Most cementable cups are more of a hemisphere than liners, and it may be harder to contain them adequately. Cementable cups generally have texturing designed to improve the fixation, whereas liners do not. Cementable cups generally do not have the same degree of options as liners, such as cross-linked PE or face changing dimensions. Although there are no clinical data on the cementing of elevated lip liners, the bench data of Bonner et al 4 suggest that these may be used with excellent fixation provided the liner is properly undersized and oriented to avoid impingement.
Preparation of the Liner
There is controversy as to whether roughening of the PE is necessary before cementing to augment the fixation strength. Modular liners have a circumferential groove for engagement of the locking mechanism, and some cementable cups have grooves created by the manufacturer. One study showed that if the replacement liner was undersized properly, the addition of grooves in the liner backside did little to improve lever out strengths. 4 The study by Meldrum and Hollis, 17 which used 4-mm undersized liners, showed that the addition of circumferential grooves in the PE improved the fixation strength of the liner. This discrepancy may be partly explained by the size of the shoulder at the base of the PE liner. Meldrum and Hollis 17 tested the HGP liner (Harris-Galante, Zimmer) which has a small shoulder, and with undersizing of 4-mm as in their test conditions, would be expected to contribute little to the rotational stability of the hemispheric PE liner. Bonner et al 4 tested the PCA liner (Porous Coated Anatomic, Howmedica, Rutherford, NJ) which has a larger shoulder, and therefore more rotational stability, and was undersized only 2mm in their test conditions. Therefore, grooves in the HGP liner might be expected to play a greater role in liner stability than in the PCA liners. Heiner et al 11 also reported better fixation strengths with a 4-mm undersized as opposed to an 8-mm undersized liner, but attributed this to differences in the cement mantle. In their tests of PE liners (Depuy) they reported that circumferential grooves improved lever out strength compared with an ungrooved liner, but the liners they compared were different shapes. All of the aforementioned authors reported better fixation strengths with the addition of circumferential as compared with cruciate grooves in the PE because the grooves are oriented perpendicular to the shear forces.
The aforementioned data indicate that in PE liners with poor intrinsic stability (small rim, no grooves for locking mechanisms) circumferential grooves may improve the fixation strength substantially. In particular, this is important when the liner may be subject to impingement stresses, such as in the case of a cemented constrained liner. The surgeon should assess the degree of intrinsic stability of the PE liner and make a decision on whether additional grooving is necessary.
If additional contouring is necessary, the surgeon should be alert to the thickness of the PE to avoid making the grooves too deep and jeopardizing the integrity of the PE. Contouring of the liner should be done on the back table because the process generates a large amount of debris. Contouring can be done with a high-speed burr using a pencil tip or a small round burr. Several studies have suggested that circumferential grooves will provide better fixation strength than cruciate patterns. 4,9,16
Preparation of the Existing Shell
It is not clear from the literature whether the metal shell needs to be roughened to improve fixation. In all of the in vitro mechanical fixation tests the failure of fixation occurred at the cement-liner interface, and not the cement-metal interface. 4,9,11,17 Similarly, the failures reported in the clinical series also have occurred at the cement-liner interface. 9,11 Nonetheless, the decision to roughen the cup should be made based on the surface finish of the inside of the cup, the presence of screw holes, and the surface characteristics of the rim locking mechanism. If the cup has a polished inner surface, LePorte et al 12 and Haft et al 9 reported that circumferentially scoring the shell may improve fixation. This scoring should be done with a high-speed burr equipped with a carbide-tipped bit or diamond cutting wheel, and as in the PE liner contouring, care should be taken to avoid weakening the metal shell by making the grooves too deep (> 2-mm).
There are no data available on the effect of different cement techniques on the long-term effectiveness of cemented PE liners. Haft et al 9 recommended that it be administered in the liquid phase to allow the dome of the liner to come to within 2 mm of the dome of the shell. Meldrum and Hollis 17 reported using doughy cement in their mechanical study. I prefer to use the cement in the early dough phase to improve pressurization into the shell, and I use a bipolar trial head (6-mm smaller than the liner) covered with a moistened surgical glove to pressurize and mold the cement. Haft et al 9 also suggested that the bone behind any screw holes be drilled to create a cement anchor effect when the liner is placed. In addition, Meldrum and Hollis 17 suggested that an important part of good cement technique is centralization of the liner within the mantle, and Bonner et al 4 emphasized centralization without allowing the liner dome to touch the metal shell. However, Haft et al 9 implied that the mantle would be incomplete by their recommendation of making the rim of the liners tight against the shell. The optimal thickness of cement mantle has not been determined. Although several authors have suggested a 2-mm mantle (therefore, a liner 4 mm less in diameter than the inner diameter of the shell) based on optimal mantles for cemented femoral and preliminary bench testing. 1,4,9,11,16 The performance of a 2-mm cement mantle subjected to cyclic loads with time in this setting is not known.
Although there is considerable bench testing data on the initial strength of fixation of a PE liner cemented into an existing shell, there are no intermediate or long-term clinical data on the performance of these constructs in vivo. Two clinical series have been reported in the literature, but the maximum followup was 7 years, and the mean followup only was 3 years. Despite success rates of approximately 90% at this followup, the use of this technique should be guarded, and the potential risks should be explained to the patient. The surgeon should be aware that there are many different shapes of PE liners, and some are considerably different than those tested in the laboratory or tested clinically. Nonetheless, in those situations where removal of a well-fixed acetabular shell poses a substantial risk to the patient, the technique of cementing a PE liner may be warranted. The surgeon should consider the age and activity level of the patient, and the durability of fixation of the acetabular shell, before proceeding with cementation of a liner. The liner size should be selected carefully to maximize containment within the metal shell. The surgeon should add additional circumferential contouring of the liner and shell if warranted, and doughy cement should be pressurized into a uniform mantle. The potential advantages of this technique are less surgical morbidity, more rapid surgery and patient recovery, the ability to incorporate antibiotics in the cement, and more liner options.
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