The ‘apple core’ cement spacer for the management of massive bone loss in two-stage revision knee arthroplasty for infection

Abstract

Antibiotic-loaded cement spacers are used in two-stage revision knee arthroplasty for infection, but commercially available spacers may not always be suitable for significant bone loss or soft tissue failure in multiply revised cases. We describe a technique for producing an on-table, static, reinforced cement spacer – the ‘apple core’ spacer – with the intended outcome of providing joint stability in such patients, prior to undertaking a second-stage procedure. Following a radical debridement, the spacer is made of three components: (1) a ‘central bar’ of external fixator connecting rods, combined using cerclage wires as needed; (2) a standard polymethylmethacrylate cement ‘apple core’; and (3) a covering ‘skin’ of high dose antibiotic-loaded cement, which is stippled as it sets, to increase the surface area and facilitate antibiotic elution. This technique was performed in nine patients who underwent two-stage salvage revision for complex, recurrent infected total knee arthroplasty at a single institution. All patients successfully went on to definitive second-stage reimplantation and have retained their limbs. The ‘apple core’ cement spacer allows massive bone defects to be effectively managed between staged revision procedures.

1. Introduction

Total knee arthroplasty (TKA) rates are increasing each year, and the prevalence of infection has risen along with the growing demand for primary and revision procedures.^1,2 Revision or re-revision surgery in the presence of chronic infection can be particularly challenging and is typically managed with debridement and one- or two-stage implant exchange.³ In two-stage protocols, a spacer acts as a local antibiotic-delivery system and provides mechanical stability to the knee.⁴

Antibiotic-loaded cement spacers can be divided into articulating (on-table mould, pre-fabricated, or interval prosthesis with metal and/or polyethylene components, such as the PROSTALAC knee system) and static designs.^5,6 The decision on what type of spacer is appropriate depends on various factors, but fundamental considerations are the condition of the soft tissues, including the extensor mechanism, and the extent of bone loss following explanation and debridement. Bone loss in revision TKA may occur pre-operatively (in the context of infection, aseptic loosening, polyethylene granulomata, metallosis, or trauma) or intra-operatively due to explantation and debridement.

Revision TKA for infection generally involves a combination of several causes of bone loss, which is often defined according to the Anderson Orthopaedic Research Institute (AORI) classification system, in which tibial (T) and femoral (F) defects are scored separately.⁷ It should be remembered, however, that this is a radiographic evaluation of metaphyseal integrity and the procedure generally ends with greater bone loss than was judged pre-operatively. It can therefore be difficult to predict the resulting defect size, and commercially available spacers may not necessarily overcome extensive bone loss.

The objective of this paper is to describe a technique for producing an on-table, static, reinforced cement spacer – the ‘apple core’ spacer – where the desired outcome is to provide joint stability in cases of multiply revised knees in the presence of significant bone defects and/or soft tissue failure, prior to undertaking a second-stage procedure. This allows the potential for mobilisation on a mechanically stable limb.

2. Surgical technique

Debridement is the most important phase in the management of infected TKA. Using principles described by Lautenbach, the soft tissues and intramedullary (IM) canals of the femur and tibia are debrided according to a cyclical protocol of curettage, reaming, and pulse lavage, after initial explantation and sharp dissection.8, 9, 10 Chemical debridement may also be performed using antimicrobial agents such as 3% acetic acid, chlorhexidine, or povidone-iodine solution.¹¹

The ‘apple core’ spacer is made of three components – a ‘central bar’, a bone cement ‘apple core’, and a covering ‘skin’ of high dose antibiotic-loaded bone cement (Fig. 1). The defect is measured in extension and with the limb held with gentle traction. Sterile external fixator connecting rods from the Hoffmann System (Stryker Corp., Kalamazoo, MI, USA) are used in combination to internally splint the defect – the ‘central bar’. Typically between 1 and 3 connecting rods of varying length can be placed within the medullary canal of the femur and tibia. Defects measuring up to 20 cm length can therefore be splinted. The overlapping rods can be held together using cerclage wires as needed (Fig. 2). Although Hoffmann external fixator rods are inexpensive and readily available in most institutions with a trauma department, other metal devices can be utilised for the same purpose, including Küntscher nails or Rush pins, depending on local availability and defect size.12, 13, 14

Fig. 1.

A schematic x-ray of the ‘apple core’ spacer, to provide stability in major bone defects, whilst also facilitating the local elution of antibiotics. It comprises a ‘central bar’ (A), a standard antibiotic-impregnated cement ‘apple core’ (B), and a high dose antibiotic-loaded cement ‘skin’ (C).

Fig. 2.

Anteroposterior (A) and lateral (B) knee x-rays of the ‘apple core’ spacer in situ following explantation and debridement of an infected revision TKA implant. Cerclage wires around the ‘central bar’ are visible. Soft tissue and bony stability determines weight bearing status.

The central bar is then covered in standard polymethylmethacrylate (PMMA) bone cement; Palacos® R+G (Heraeus Medical GmbH, Wehrheim, Germany), which already contains gentamicin, is used at our institution for this purpose. This is applied around the central bar and pressed into the IM canals of the femur and tibia to maintain defect spacing with adequate soft tissue tension (Fig. 3). This ‘apple core’ is allowed to set firmly around the central bar and within the first few centimetres of bone, thus stabilising the bone interfaces and soft tissue envelope.

Fig. 3.

The ‘central bar’ comprised of Hoffmann external fixator connecting rods covered with standard antibiotic-impregnated cement – the ‘apple core’ – is allowed to set firmly, maintaining stability and soft tissue tension.

Finally, the ‘apple core’ is covered with a layer of high dose antibiotic-loaded bone cement – the ‘skin’ (Fig. 4). This final layer introduces a high concentration of local antibiotics into the operative field. Based on the results of microscopy, culture and sensitivities, and with advice from microbiology, specific antibiotics can be added to standard cement, or pre-mixed dual antibiotic bone cement may be used, such as Vancogenx® (Tecres S.p.A., Sommacampagna, Verona, Italy) at our institution. As the cement ‘skin’ sets, the outer layer is stippled (Fig. 5) with a MacDonald dissector to increase the antibiotic elution.¹⁵

Fig. 4.

The ‘apple core’ with application of high dose antibiotic-loaded cement ‘skin’.

Fig. 5.

End result with stippled surface of the cement ‘skin’ to enhance local antibiotic elution.

Post-operative weight bearing status can be judged according to the bone interface stability and soft tissue tension. It is recommended, however, that weight bearing is limited and supported by an external brace or splint. The length of time between first and second-stage procedures should be individualised for each patient based on age, co-morbidities, pathogenic organism(s), antibiotic requirements, and soft tissue healing.

3. Clinical cases

Between August 2015 and December 2017, the senior author (RMJ) performed this technique in nine patients for two-stage salvage revision in complex, recurrent infected TKA. All of these individuals had T3/F3 bone defects according to the AORI classification system.⁷ Following explantation and intra-operative tissue sampling at first stage, dual broad-spectrum intravenous antibiotics were commenced as per microbiology advice. Patients were treated in hospital until the results of extended cultures were available, after which appropriate antibiotics (oral and/or intravenous) were commenced and patients discharged home with an outpatient clinic review at fortnightly intervals, or earlier, as dictated by their clinical condition and appearance of the wound and soft tissues. Partial weight bearing was permitted in cases where the stability of the bony interfaces was sufficient.

Satisfactory wound healing with pain relief, stable observations, and the decline of inflammatory markers were indications for undertaking the second-stage procedure which, in these salvage cases, might require a megaprosthesis or implant arthrodesis. We experienced no issues with wound closure or wound healing, as the diameter of the spacer was set within the soft tissue envelope at the time of cementation (Fig. 3). All nine patients in our series went on to definitive second-stage revision and have retained their limbs at a minimum follow-up of 2 years. Four patients with intractable extensor mechanism failure underwent arthrodesis with a cemented coupled nail (Waldemar LINK GmbH and Co, Hamburg, Germany), whilst the remaining five underwent megaprosthetic reconstruction with Limb Preservation System components (LPS™; DePuy Synthes, Warsaw, IN, USA).

4. Discussion

Radical debridement and stability of soft tissues and bone interfaces is paramount in achieving eradication of periprosthetic joint infection and wound healing, irrespective of whether one- or two-stage exchange is preferred.¹¹ The pre-requisites for an articulating spacer are an intact extensor mechanism, intact collateral ligaments, adequate soft tissue coverage, adequate bone stock, and a compliant patient. If any of these factors are absent, a static spacer is indicated, as is the case for many patients with a multiply revised TKA. Whether articulating or static, however, spacers should provide soft tissue stability, eradicate bone interface movement, prevent secondary injury to soft tissues and bone stock, optimise the interval between stages, and facilitate joint reconstruction at the second-stage. Weight bearing between stages is certainly desirable but needs to be assessed for each individual case, to minimise the risk of instability, spacer dislocation or breakage. Fehring et al. have also emphasized the importance of resting the joint adequately in infected and inflammatory conditions.⁶

Patients with massive bone loss following multiply revised TKA or revisions of tumour implants for infection present a further challenge. In these scenarios, different techniques may be required to create bespoke static, reinforced cement spacers, such as the ‘apple core’ spacer. However, there are relatively few reports in the literature regarding options for overcoming extensive bone loss between stages of revision for infection. Antoci et al. have described using antibiotic-loaded cement around a rod-spacer,¹⁶ whilst Nickinson et al. have used a Küntscher nail which is only partially covered in cement, but extends slightly further into the femoral and tibial intramedullary canals.¹² Static cement spacers can also be reinforced with a long intramedullary nail, inserted antegrade through the piriformis fossa, in cases where significant bone loss or ligamentous instability prevent the formation of a stable joint with the conventional spacer alone,¹⁷ but this is a lengthy and technically demanding procedure, which may be preferable as definitive treatment.

Antibiotic-loaded bone cement can be either commercially bought or custom prepared on-table. Many of the standard antibiotic-impregnated cements have a comparatively low dose of antibiotic, which can act as prophylaxis in sterile arthroplasty surgery, but are less likely to be efficacious in the context of established infection. Therefore, additional antibiotics can be added to the cement to achieve high local concentrations for the treatment of periprosthetic joint infection and to tailor the drug to the causative organism(s).¹⁸

We used a pre-mixed cement containing both gentamicin and vancomycin for the covering ‘skin’, but Frew et al. have shown that high elution rates can be achieved in vitro when adding vancomycin powder manually to standard gentamicin-containing bone cement.¹⁹ Although there are significant potential cost savings, the elution profile and mechanical properties of manually mixed cement are less predictable than commercial preparations.^19,20

It has also been demonstrated in vitro that it is possible to change the surface properties of PMMA cement in order to improve antibiotic elution.^15,21 Indenting the outer layer with a MacDonald dissector achieves this by increasing the surface area to volume ratio. An alternative method of delivering high doses of local antibiotics is the use of absorbable synthetic calcium sulphate beads (Stimulan®; Biocomposites Ltd, Keele, UK), which has gained interest in the treatment of periprosthetic joint infections, as it is suitable for both one- and two-stage revision procedures, and can be mixed with heat-labile antibiotics – unlike PMMA beads.^22,23

In our technique, using external fixator rods means that the length of the ‘central bar’ can be decided on-table immediately before applying the ‘apple core’ layer of cement, which in turn sets and achieves adequate joint stability and soft tissue tension, prior to the covering ‘skin’ of high dose antibiotic-loaded cement. Although it was only in a limited cohort of patients with complex, infected re-revision TKA that we have undertaken this technique to date, the intended outcome was achieved in all cases, and the same principles may be useful in two-stage revision protocols for infected tumour endoprostheses around the knee.

5. Conclusion

We have found that the ‘apple core’ spacer is a viable option in cases of significant bone loss, providing stability, maintaining leg length, potential weight bearing and mobility, enhancing the elution of local antibiotics, and allowing relative ease of removal at second-stage, all of which are valuable in facilitating definitive reconstruction.

Funding

None.

Contributions

NR and RK wrote and revised the article. RMJ developed the technique, performed the cases, and revised the article. Each co-author has contributed equally to this publication.

Declaration of competing interest

The authors have no conflicts of interest to declare.