Stem compatibility for cement-in-cement femoral revision: an in vitro study

Introduction

Since its original description by Eftekhar in 1978 (1), cement-in-cement revision arthroplasty of the hip remains a viable technique for femoral component revision when a good quality cement mantle and bone-cement interface are present. This technique is particularly attractive for the treatment of acetabular component failure, where removal of the femoral component may facilitate surgery. Others also advocate its use for conversion of a cemented hemiarthroplasty to total hip arthroplasty, altering femoral anteversion in cases of recurrent dislocation, for selected periprosthetic fractures (2–3–4), and in cases of revision for infection (5, 6).

Greenwald et al validated this technique biomechanically by demonstrating only a 6% decrease in shear strength at the new/old cement interface, after rasping and drying, when compared with a uniform block of cement (7). The shear strength only drops by 33% when the two layers are interposed with blood (7). The effect of fluid interposition, and need for cement roughening is further downplayed by Rudol and colleagues, except when a bone marrow mimicking fluid was interposed between the layers of cement causing a dramatic reduction in shear strength (8). Rosenstein et al have showed that in all cases the shear strength of the cement-cement interface was greater than that of the cement-bone interface (9). To date, mixing brands of cement for the purpose of cement-in-cement revision does not appear to cause any complications clinically (10) or biomechanically (11).

Clinically this technique has been validated in a series of 136 Exeter stem cement-in-cement revisions with five- to 15-year follow-up, in which no femoral re-revisions for aseptic loosening were necessary (10). Other encouraging results have been reported by centres using a variety of femoral components (5, 12–13–14–15).

Cement removal can be performed with ultrasonic tools (16, 17), high speed burrs, osteotomes, reamers, trochanteric osteotomy, controlled femoral perforation (18), endoscopically (19) and by computer assisted fluoroscopic navigation (20). These techniques are either costly, onerous or challenging, and associated with the risk of femoral perforation (21–22–23) and periprosthetic fracture (23, 24).

In 2006, a slender Exeter V40 (Stryker, Newbury, UK) short femoral stem with a 44 mm offset was marketed for use in cement-in-cement revision (25). We identified one case report of this stem fracturing after cement-in-cement revision in a patient with a BMI of 27.8, despite neutral stem alignment and a good proximal cement mantle (26). The US department of Food and Drug Administration (FDA) has received additional adverse event reports regarding fracture of this slender Exeter short femoral stem (27). The same problem has been documented for the Corin (Corin Group PLC, Cirencester, UK) Taper-Fit CDH stem after its use for cement-in-cement revision (28).

To our knowledge, this is the first study that aims to establish which femoral components seat into other cement mantles without the need for alteration. The secondary aims were to quantify the variation in stem height (i.e. leg length when in vivo) and anteversion afforded by the new femoral component.

Materials and Methods

We selected four of the five most commonly implanted cemented femoral components according to the 10^th Report from the National Joint Registry of England and Wales (2013) for inclusion in this study (29). These components are the Exeter V40 (Stryker, Newbury, UK), CPT (Zimmer, Warsaw, Indiana), C-stem (DePuy, Leeds, UK) and the Charnley (DePuy, Leeds, UK). We tested the range of Exeter stem sizes with 37.5 mm offset and 44 mm offset, as well as the 35.5 mm Asian Pacific and the 44 mm slender short Exeter designed for cement-in-cement revision. The C-stem was tested but the newer C-stem AMT with a raised lateral shoulder was not. The range of CPT sizes and a selection of Charnley stems were also tested. These stems were obtained from implant retrieval if they were in good condition. Larger stem sizes (CPT 5 and C-stem (6–7–8–8)) were not tested as they are rarely used in our institution.

A testing rig was developed using a fashioned polypropylene pipe sealed inferiorly to represent the femoral canal. The pipe was then filled with dental cement (Ultrahard Die Stone Iso-Type IV, Kerr S.p.A, Italy), which had been mixed and prepared according to manufacturer's instructions. After two minutes the stems were lowered into the centre of the cement mantle by the rig, to the level of the middle line, or marking on the implant. The stem was then held in situ with a clamp until the cement had cured, thus eliminating micromotion or varus/valgus malalignment.

After the cement had cured, a screw was inserted through the pipe and into the cement mantle parallel with and inferior to the axis of the trunion. This fixed point was used for the measurement of stem height. A finer pin was inserted through the side of the pipe and into the cement mantle at approximately 90 degrees to the trunion as a fixed reference point for the measurement of stem anteversion (Fig. 1). The stem height was recorded as the distance between the inferior screw and the most proximal tip of the trunion, using callipers accurate to 0.1 mm (Fig. 2). Due to small variations in trunion diameter, we acknowledge that the most proximal tip of the trunion is a surrogate for stem height. Stem version was assessed by obtaining a photograph from directly above the mounted stem and subsequently measuring the angle between the pin and the orientation of the stem in its own mantle. Photographs were standardised by mounting the camera in a clamp at a fixed height of 26 cm above the test table, with a fixed position for the mounted stem in the polypropylene pipe to ensure that it was consistently centred below the midpoint of the camera lens.

OPEN IN VIEWER

Fig. 1

Superior photograph of femoral stem with anterior fine wire shown.

OPEN IN VIEWER

Fig. 2

Technique utilised for assessment the stem height within a known cement mantle.

Each stem was then removed from its original cement mantle. Occasionally this required the use of an osteotome to remove a small amount of cement obstructing the shoulder of the stem. Sequentially, all the stems were inserted into each cement mantle. Stems were divided into those that seated below the original stem, those that seated at the same height, those that seated between 0-10 mm high, 10.1-20 mm high, and those that would not seat at all.

Any stem causing greater than 20 mm of increase in stem height was deemed grossly incompatible and no further measurements (either stem height or version) were obtained. Such incompatibility is indicated by a dash (-) in results Table I. However if a stem could be inserted into the test cement mantle resulting in no more than 10 mm of additional stem height, then a full assessment of the new stem height and version was conducted in the assumption that in vivo, seating may occur after a little cement removal. This is a true reflection of this type of revision surgery, as some cement preparation is often needed. Marginal compatibility is indicated by orange shading in results Table I. Stems which readily seated without an increase in stem height were deemed fully compatible, and again a full set of measurements were taken. These fully compatible stems are shaded green in results Table I.

OPEN IN VIEWER

TABLE I

Revision Stem Compatibility - Stem heights

Cement mantle		Exeter CiC revision 44 mm offset	Exeter 35.5	Exeter 37.5						Exeter 44						C - Stem					CPT				Charnley
Revise to			35.5	CDH	0	1	2	3	4	0	1	2	3	4	5	1	2	3	4	5	1	2	3	4	FB45	FB40	RB45
Exeter CiC revision 44 offset stem		0.0	12.0	4.9	5.6	5.7	2.2	-0.5	-2.6	0.9	-0.6	-2.7	-5.8	-7.7	-7.4	7.7	2.3	-8.4	-4.0	-11.1	13.6	4.7	3.9	0.7	1.3	-1.1	-5.6
Exeter 35.5	35.5	5.6	0.0	-4.8	-0.4	-3.6	-8.7	-4.6	-12.0	-3.7	-11.4	-11.8	-15.7	-13.3	-17.7	7.8	0.0	-11.3	-9.9	-11.9	4.8	-3.9	-0.9	-8.1	-3.8	-7.9	-5.3
Exeter 37.5	CDH	–	–	0.0	9.2	4.3	3.4	6.3	1.1	13.4	4.8	7.5	5.4	15.5	7.7	–	–	12.9	15.9	15.4	–	18.9	15.3	19.9	–	18.9	18.8
	0	–	–	1.3	0.0	4.2	2.3	5.5	1.1	12.9	5.6	6.1	7.9	14.1	7.6	–	–	13.8	15.6	12.6	–	18.3	14.5	18.7	–	18.2	–
	1	–	–	–	–	0.0	7.5	8.7	0.2	–	14.4	14.5	14.4	11.3	4.0	–	–	14.5	17.3	11.8	–	–	15.4	15.6	–	–	–
	2	–	–	–	–	–	0.0	4.0	0.0	–	–	15.9	11.2	11.5	4.1	–	–	–	–	14.6	–	–	13.3	–	–	–	–
	3	–	–	–	–	–	13.7	0.0	0.6	–	–	–	13.3	10.2	0.9	–	–	–	–	–	–	–	–	–	–	–	–
	4	–	–	–	–	–	–	–	0.0	–	–	–	–	15.9	10.6	–	–	–	–	–	–	–	–	–	–	–	–
Exeter 44	0	–	–	5.1	16.3	7.5	1.9	2.2	0.7	0.0	2.2	1.7	1.9	4.5	1.1	–	–	13.7	14.3	11.0	–	12.8	16.8	10.4	–	–	19.5
	1	–	–	–	–	9.6	3.1	1.5	0.0	–	0.0	4.8	5.7	8.5	0.5	–	–	11.5	12.9	10.2	–	16.8	10.0	10.2	–	–	18.1
	2	–	–	–	–	19.0	4.4	1.9	0.0	–	10.3	0.0	7.6	7.9	0.5	–	–	16.2	18.4	8.9	–	–	–	12.1	–	–	16.8
	3	–	–	–	–	–	11.8	7.1	4.0	–	–	–	0.0	10.1	5.0	–	–	–	–	17.8	–	–	–	14.7	–	–	18.4
	4	–	–	–	–	–	–	12.4	5.9	–	–	–	12.3	0.0	1.5	–	–	–	–	–	–	–	–	–	–	–	–
	5	–	–	–	–	–	–	–	7.1	–	–	–	17.3	9.4	0.0	–	–	–	–	–	–	–	–	–	–	–	–
C-Stem	1	–	–	19.8	–	9.4	0.8	-5.5	-6.0	–	-1.9	-1.9	0.0	-7.8	-14.0	0.0	-2.1	-13.3	-11.2	-15.3	–	10.4	9.3	-1.6	16.3	17.2	1.9
	2	–	–	–	–	11.7	4.1	0.0	-2.6	–	9.5	10.1	11.4	2.2	-4.1	–	0.0	-10.2	-6.7	-11.6	–	16.3	–	6.1	–	–	10.1
	3	–	–	–	–	–	18.6	20.0	1.7	–	–	–	–	15.1	10.4	–	17.4	0.0	-3.6	-7.3	–	–	–	–	–	–	–
	4	–	–	–	–	–	–	–	11.0	–	–	–	–	–	–	–	–	7.4	0.0	-0.5	–	–	–	–	–	–	–
	5	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	13.3	11.1	0.0	–	–	–	–	–	–	–
CPT	1	–	–	–	–	-2.7	-3.8	-2.8	0.0	19.5	-3.7	-2.6	-1.4	-3.1	-2.0	–	19.1	6.4	10.1	9.6	0.0	2.7	10.1	2.1	–	14.5	14.9
	2	–	–	–	–	0.5	-2.4	-2.0	2.3	19.9	-3.0	-1.7	-1.8	-0.9	-2.1	–	–	8.5	10.4	7.7	23.9	0.0	7.6	3.4	–	15.1	14.0
	3	–	–	–	–	14.4	-0.3	-3.0	-2.1	–	7.9	1.3	-5.5	-4.0	-3.7	–	–	7.7	12.1	6.5	–	19.0	0.0	2.5	–	13.5	13.6
	4	–	–	–	–	–	5.6	1.4	-1.6	–	18.9	13.1	8.3	-1.7	-3.8	–	–	14.9	–	11.2	–	–	–	0.0	–	–	13.1

Orange: Stem may seat easily with some cement removal (new stem sits proud by 0.1-10 mm).

Yellow: Restoration of original stem height (0 mm).

Green: Revision stem seats easily (new stem height <0 mm).

Dash: Stem would not seat within 20 mm of original height.

To assess the possible additional version which can be afforded by a new stem in a cement mantle, the new stem was positioned at the original stems’ height (distance between the inferior screw and the tip of the trunion), positioned in maximum anteversion and held with Blu-Tack® before a second photograph was obtained from above the rig. Stem anteversion was measured from the photographs by one observer (AT). Test stems that increased the stem height had their anteversion tested in the position of maximal seating within the foreign cement mantle. Sequentially all stems were trialled in each of the test cement mantles. Test stems that produced >5° increase in anteversion, are shaded in green in Table II, stems offering a 3-4° increase in anteversion are shaded in yellow and those producing 2° or less of anteversion are shaded in orange. Although no stems were retroverted, due to small inaccuracies in measurement, some stems record a small negative value for anteversion.

OPEN IN VIEWER

TABLE II

Revision Stem Compatibility - additional stem anteversion

Cement mantle		Exeter CiC revision 44 mm offset	Exeter 35.5	Exeter 37.5						Exeter 44						C - Stem					CPT				Charnley
Revise to			35.5	CDH	0	1	2	3	4	0	1	2	3	4	5	1	2	3	4	5	1	2	3	4	FB45	FB40	RB45
Exeter CiC revision 44mm offset		0.0	0.0	1.0	-0.5	10.0	6.0	17.0	14.0	-2.0	4.0	3.0	2.0	6.0	11.0	6.0	3.0	5.0	2.0	19.0	0.0	1.0	1.0	5.0	2.0	2.0	9.0
Exeter 35.5	35.5	0.0	0.0	1.0	0.0	11.0	9.0	9.0	18.0	0.0	6.0	6.0	6.0	23.0	34.0	20.0	0.0	36.0	18.0	34.0	-3.0	1.0	1.0	3.0	-2.0	19.0	15.0
Exeter 37.5	CDH	–	–	0.0	1.0	10.0	1.0	8.0	11.0	–	6.0	9.0	0.0	–	20.0	–	–	–	–	–	–	–	–	–	–	–	–
	0	–	–	–	0.0	10.0	5.0	9.0	11.0	–	9.0	7.0	6.0	–	17.0	–	–	–	–	–	–	–	–	–	–	–	–
	1	–	–	–	–	0.0	-2.0	-1.0	7.0	–	–	–	–	–	8.0	–	–	–	–	–	–	–	–	–	–	–	–
	2	–	–	–	–	–	0.0	1.0	5.0	–	–	–	–	–	10.0	–	–	–	–	–	–	–	–	–	–	–	–
	3	–	–	–	–	–	–	0.0	2.0	–	–	–	–	–	7.0	–	–	–	–	–	–	–	–	–	–	–	–
	4	–	–	–	–	–	–	–	0.0	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–
Exeter 44	0	–	–	0.0	-1.0	11.0	5.0	4.0	16.0	0.0	4.0	6.0	7.0	9.0	12.0	–	–	–	–	–	–	–	–	–	–	–	–
	1	–	–	–	–	6.0	4.0	0.0	7.0	–	0.0	2.0	-1.0	4.0	12.0	–	–	–	–	–	–	–	-1.0	–	–	–	–
	2	–	–	–	–	–	0.0	-2.0	5.0	–	–	0.0	0.0	8.0	13.0	–	–	–	–	–	–	–	–	–	–	–	–
	3	–	–	–	–	–	–	1.0	4.0	–	–	–	0.0	–	10.0	–	–	–	–	–	–	–	–	–	–	–	–
	4	–	–	–	–	–	–	–	2.0	–	–	–	–	0.0	10.0	–	–	–	–	–	–	–	–	–	–	–	–
	5	–	–	–	–	–	–	–	1.0	–	–	–	–	4.0	0.0	–	–	–	–	–	–	–	–	–	–	–	–
C Stem	1	–	–	–	–	13.0	1.0	21.0	17.0	–	5.0	7.0	2.0	13.0	24.0	0.0	3.0	15.0	12.0	17.0	–	–	1.0	6.0	–	–	5.0
	2	–	–	–	–	–	4.0	7.0	7.0	–	7.0	–	1.0	8.0	12.0	–	0.0	13.0	10.0	18.0	–	–	–	0.0	6.0	–	–
	3	–	–	–	–	–	–	–	6.0	–	–	–	–	–	–	–	–	0.0	4.0	4.0	–	–	–	–	–	–	–
	4	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	9.0	0.0	4.0	–	–	–	–	–	–	–
	5	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	–	0.0	–	–	–	–	–	–	–
CPT	1	–	–	–	–	4.0	4.0	16.0	–	–	3.0	1.0	–	11.0	12.0	–	–	34.0	–	–	0.0	-2.0	–	-2.0	2.0	–	–
	2	–	–	–	–	7.0	3.0	16.0	12.0	–	4.0	4.0	0.0	10.0	14.0	–	–	23.0	–	34.0	–	0.0	-1.0	-1.0	–	–	–
	3	–	–	–	–	–	0.0	17.0	10.0	–	4.0	2.0	1.0	8.0	15.0	–	–	10.0	–	–	–	–	0.0	1.0	–	–	–
	4	–	–	–	–	–	1.0	12.0	4.0	–	–	–	-3.0	4.0	12.0	–	–	–	–	–	–	–	–	0.0	–	–	–

Orange: Limited change in anteversion (-2° to +2°).

Yellow: Small increase in anteversion (3° to 4°).

Green: Large increase in anteversion (>5°).

Dash: Stem would not seat within 10mm of original height - anteversion not assessed.

In total 26 stems were used in this study, resulting in 650 stem and mantle combinations. We did not attempt to use the Charnley stems as revision options, reflecting common clinical practice. This resulted in 572 stem and mantle combinations being tested.

Results

Full seating occurred in 105 of the stem and mantle combinations, and 205 combinations allowed seating with less than 10 mm of lengthening. Of all the standard sizes tested, the size 1 C-stem demonstrated the greatest overall compatibility being fully seated in 12/25 (48%) of the cement mantles. The only stem which fully seated in more cement mantles was the tiny Exeter 35.5 mm (21/25 84%) Asian Pacific stem, but we accept this is rarely used in European patients.

When revision stems were considered, the slender 44 mm offset Exeter stem designed for cement-in-cement revision was also fully seated in 12/25 (48%) of mantles.

Of the five stem systems tested in this study, only the C–stem consistently demonstrated satisfactory seating when downsizing, i.e. a smaller size of C-stem always seated in a larger C-stem mantle. This was not the case for the Exeter 37.5 mm offset, Exeter 44 mm offset or the CPT systems. Paradoxically, downsizing within these designs of stem caused an increase in stem height (effectively leg lengthening when in vivo) in 14/15 (93%), 15/15 (100%) and 6/6 (100%) of cases respectively.

The CPT system proved to be a useful implant as a revision option for both the Exeter 37.5 mm offset and Exeter 44 mm offset systems with 25/44 (56.8%) of CPT stems seating at or below the height of the original Exeter stem without any refashioning to the original Exeter cement mantle. However, none of the CPT stems tested could seat into the Exeter size 0 mantle (37.5 mm or 44 mm offset).

The size 1 C-stem was similarly versatile for revising into an Exeter cement mantle, being well seated in 7/11 (63.6%). As with the CPT system, no C-stem seats into the Exeter size 0 mantle without significant cement removal. Above size 1 C-stem, compatibility within other mantles decreases with only 3/33 (9.1%) being well seated.

There were notably few revision options for a CPT mantle. Apart from the Exeter 35.5 stem which seated easily into CPT sizes greater than size 1, the C-stem size 1 was the only stem to be compatible with a CPT mantle, and this was limited to the size 4 CPT mantle only.

We observed that stems that seat easily also tend to allow for additional anteversion. One hundred and fifty-five (27.1%) of the 572 revision stem options that were trialled offered an increase in stem anteversion (range 1-36°). In total 103 (18.0%) revision stem options offered a relative increase in anteversion of between 5-36° from the original host stems’ version. However, of the 105 revision options that fully seated, 75 (71.4%) offered an increase in anteversion.

When downsizing an implant within the same system, the greatest utility for increasing stem anteversion by 5° or more were the Exeter 35.5 mm offset stem, the Exeter cement-in-cement 44 mm offset revision stem and size 1 C-stem, which allowed for an increase in version in 9/12 (75%), 6/12 (50%) and 3/4 (75%) cases of mantles respectively. Downsizing within the CPT system did not afford any additional anteversion.

The stems that afforded the largest gains in anteversion (greater than 5°) were the Exeter 35.5 mm offset stem, the size 1 C-stem, and the Exeter 44 mm offset cement-in-cement revision stem, in 15/25 (60%), 12/25 (48%), and 11/25 (44%) of mantles respectively.

Discussion

In this study, we provide in vitro evidence of the relative compatibility of various stems in other cement mantles for cement-in-cement revision. This allows surgeons to choose revision implants which will seat easily, and allow for alterations in stem height and version where necessary. This may be of most value when revising unfamiliar stems. It appears that the cement mantle can seldom be left without some technical adaptation to accept the new stem.

As predicted the smallest stems tested in this study were the most compatible with the greatest majority of the foreign cement mantles. Unfortunately many of these small stems either do not offer enough offset (e.g. Asian Pacific 35.5 mm Exeter stem), or have been associated with stem fracture (26, 28).

The C-stem system affords distinct advantages over the other systems for the purpose of cement-in-cement revision. It is the only system tested in which smaller sizes predictably seat within larger sized cement mantles. The C-stem size 1 is also a useful stem for revision into most Exeter and all C-stem mantles and with some reshaping also the CPT mantles (but only fits into the size 4 CPT mantle without reshaping), avoiding the use of the slender and fracture prone high offset stems designed for cement-in-cement mantles.

Downsizing within other systems did not result in predictable seating of the ‘smaller’ sized stem. This is particularly problematic within the Exeter system. We advise significant caution when downsizing within this system as a result. It appears that the proximal stem geometry of the Exeter system is not uniformly graduated as size increases. Even downsizing by several sizes does not guarantee easy seating of the new stem. Dropping from 44 mm to 37.5 mm offset, within the same size paradoxically causes lengthening. These unusual characteristics are not discussed in the product literature.

The CPT stem system is shorter than the Exeter system, making it a good option in cases where the original cement mantle is an Exeter, and may provide surgeons with the option of using a larger stem thereby reducing the risk of stem fracture. The shortness of the CPT stem means that revising into a CPT cement mantle would usually require significant removal of the distal cement mantle.

Although the ease of stem removal from its own cement mantle was not measured, anecdotally the easiest stem to remove was the CPT, closely followed by the Exeter stem. The CPT stem has a high and straight lateral shoulder that results in no cement having to be removed from the shoulder region prior to extraction. Although the Exeter stems were also easy to remove, the rounding that occurs at the shoulder meant that some cement impeded explantation. The original C-stem, with its low lateral shoulder was harder to remove. It required some cement removal from the shoulder region. The design changes that occurred with the C-stem AMT (higher and straight lateral shoulder) theoretically make these easier to remove from their cement mantles than the original C-stem. The Charnley stems were extremely difficult to extract from their mantles, necessitating significant destruction of the proximal cement mantle.

In this study 18.0% of revision stem options trialled, when restored to the original host stem height, allowed for a change in version of 5° or more. The stems that had the greatest utility for this were the Exeter 35.5 mm offset stem, the Exeter cement-in-cement 44 mm offset revision stem and the size 1 C-stem.

This bench top study has several limitations. Removal of stems sometimes necessitated a small but unquantified removal of cement from the shoulder region which may affect the seating or anteversion of other stems. However, this is a true reflection of this type of surgery. We also note that stem anteversion occasionally measures -2°, which represents a small but systematic measurement error of at least 2° because retroversion should only be possible if anteversion is also, and most likely reflects no real change in anteversion.

Long term in vivo changes of the cement mantle are not taken into account by this study. Posterior head migration caused by joint reaction forces may cause polished stems to have a non-rectangular void in the cement mantle, potentially limiting the applicability of the results of this study. We present compatibility charts for cement-in-cement revision of the leading UK stems. The height and version data tables we present should be used in parallel to augment preoperative planning, and potentially minimise arduous refashioning of the original mantle. Reshaping of thin mantles in cases where the femoral canal is tight is precarious and the data presented herein guides surgeons to revision options which avoid the requirement for reshaping. Furthermore, data concerning alteration of anteversion may be of some use in cement-in-cement revision cases that are being performed for recurrent dislocation. Where possible we advocate that the use of normal sized stems for cement-in-cement revision to reduce the risk of stem fracture that exists with the slender cement-in-cement revision designs. Of the normal sized stems tested, overall the C-stem size 1 provides the surgeon with the most revision options, and the C-stem system affords the greatest compatibility with itself.