A total knee replacement (TKR) is a successful surgical treatment for patients with end-stage arthritis. The national joint registries for the UK estimate that more than 63,000 replacements of the knee were carried out in 20091,2. Although the clinical results of primary TKR continue to be excellent, the number of revision procedures required is increasing1,2. In the UK alone 3,798 revisions were performed between 2007/082.
The goals of revision arthroplasty are pain relief, restoration of the anatomical alignment and functional stability and accurate re establishment of the joint line. These revision arthroplasties can be challenging with the need to deal with bone defects, ligament stability and fixation. Careful selection of the appropriate implant is important and should be based on the severity of bone loss and on the status of the ligamentous and soft tissue stabilising structures.
Several systems of classification have been proposed for assessing bone loss prior to revision. The most commonly-used is that of the Anderson Orthopaedic Research Institute (AORI) which classifies the femur (F) and tibia (T) separately3. Type 1 defects are those with an intact cortical rim with localised metaphyseal defects which will not compromise the stability of a revision component. In type 2 there is damaged metaphyseal bone with loss of cancellous bone in the metaphyseal segment. Defects can occur in one femoral condyle or tibial plateau (2A) or in both condyles or plateaux (2B). Finally in type 3 there is deficient metaphyseal bone and cortical rim. Bone loss comprises a major portion of either condyle or plateau. These defects are occasionally associated with detachment of the collateral or patellar ligaments and usually require long-stemmed revision implants with bone grafts or a custom-made hinged prosthesis.
Ideally, the revision knee replacement system will offer the options of adjunctive stem fixation, methods to manage bone loss and various levels of prosthetic constraint. Current treatment options for tibial and femoral bone loss in the revision setting include cement, morselized or structural allograft, metal wedges and augments, and custom or hinge/tumour prostheses. The use of stem extensions and offset stems can assist with component position, supplement fixation, decrease stress at the bone-implant interface, and help address asymmetric bone defects. There are currently no comparative studies to guide the revision surgeon with regards to choice of revision option in managing bone loss. The aim of this article is to describe a relatively new technique in revision knee arthroplasty which specifically addresses large metaphyseal defects which can be associated with both complex primary knee arthroplasty and during revision knee arthroplasty.
The mobile bearing tray revision system
During the past 2 years we have been using the mobile bearing tray (MBT) revision knee system with metaphyseal sleeves (DePuy Orthopaedics, Warsaw, USA) in all of our revision cases and in complex primaries associated with significant bone loss. The MBT revision tray provides a stable and versatile foundation in the revision knee setting by offering a wide array of intraoperative options to assist the revision knee surgeon when handling bony and soft-tissue deficiencies. There are multiple sizes of tibial components to allow for proximal tibial coverage. Stepped metaphyseal sleeves allow for the filling of bony defects and superior metaphyseal compressive loading. Trial sleeves are sequentially broached until bony defects are overcome and solid fixation in the metaphyseal bone is achieved. Tibial augmentations are also available to manage uncontained bony defects, allowing the surgeon to achieve a stable platform on good bone for excellent fixation. Cemented or uncemented tibial diaphyseal stems are available in various lengths and diameters. The mobile-bearing tibial revision tray allows the revision surgeon to accomplish the goals of filling substantial bony defects, restoring the joint line, and providing a strong platform for solid fixation with compressive loading of bone.
|Figure 1: The metaphyseal sleeve component. B. Sleeve incorporated in the MBT tibial component (Printed with permission from DePuy Orthopaedics, Warsaw, USA)
Stepped metaphyseal sleeves (Figure 1) are a relatively new concept in the field of revision knee surgery to help modulate bony deficiency. These implants can be used in a cemented or cementless manner to manage extensive defects in the proximal tibia or distal femur. Traditionally, these large bony defects were managed with allografts. However concern regarding the availability of proper sized allografts, disease transmission, long-term fixation and incorporation of the graft has rendered them unfavourable. As a result most revision surgeons have moved from allograft reconstruction of large bony defects to metal augmentations, cones or sleeves as they have become available. The indications for metaphyseal sleeves are enlarged contained or uncontained metaphyseal defects that would otherwise require an allograft (Figure 2).
|Figure 2: Radiograph showing large uncontained defects within the femur and tibia.
They are particularly helpful in situations in which rotational control is compromised with conventional implants. They achieve this rotational control by engaging the metaphyseal cortical bony surface. Unique tibial and femoral stepped sleeves compensate for cavitatory defects, compressively load the metaphyseal bone avoiding excessive bone resection and provide a solid foundation for implant stability.
The main advantages of metaphyseal sleeves are that they are easier to use as compared to structural bone allografts leading to lower operative times and allow immediate weight bearing. The concerns are that they do not restore bone stock, and when used in a cement less manner the fit is not intimate but rather point contact with resultant gaps4.One disadvantage of using the sleeves may be the difficult removal of the implants once they have osteo-integrated. We are unable to comment upon this potential difficulty as extraction of these sleeves has not been required so far.
The following is a brief description of the surgical technique that we currently employ when using this revision tray system. All procedures have been carried out by the 2 senior authors (IJB and PW).
Revision total knee arthroplasty begins with thorough clinical and radiography evaluations. Physical evaluation includes examining the soft tissues and noting previous incisions. Range of motion, ligamentous stability and integrity of the extensor mechanism are evaluated. Patients are also requested to complete functional assessment questionnaires. Radiographic evaluation includes weight bearing biplanar views of the knees and tangential views of the patella to assess the present implant and evaluate bone stock, its alignment as compared to the extremity, location of the anatomic joint line and evaluation of the tibial and femoral bone stock. We do not routinely obtain full length radiographs but these can be helpful in assessing overall alignment. Radiological assessment using standard anteroposterior and lateral views is known to underestimate bone loss5 and therefore in all cases we ensure that allograft is available.
When possible, we prefer to use the scar from the primary procedure. Where parallel incisions are present, the more lateral is usually preferred, because the blood supply to the extensor surface is medially dominant. Fibrous adhesions are carefully released to re-establish the suprapatellar pouch and medial and lateral gutters Where patellar mobilisation difficulties persist, a quadriceps snip, a proximal inverted quadriceps incision (modified V-Y), or a tibial-tubercle osteotomy may be indicated. Multiple intra-operative cultures are obtained at every revision.
Extraction of Implants
Care is taken to preserve as much bone as possible. We use a selection of tools including thin osteotomes, an oscillating saw, a Gigli saw and various extraction devices. The bone/cement or bone/prosthesis interface is carefully disrupted before extraction is attempted. The implanted components are disengaged and extracted as gently as possible to avoid fracture and unnecessary sacrifice of bone stock. When the entire prosthesis is to be replaced, we remove the femoral component first, because this will enhance access to the proximal tibia. All residual bone cement is cleared.
Accurate assessment of the quantity and location of remaining cortical and cancellous bone can now be accurately graded using the AORI system3 and considered in the final assessment of whether augments and sleeves are required to supplement the revision. The most common scenario that we have encountered in our series is a large uncontained osseous defect of the medial tibial plateau with varying amounts of the lateral tibial plateau remaining for structural support.
For purposes of alignment, we prepare the proximal tibial with reference to the position of an intramedullary (IM) reference guide. The location of the medullary canal is approximated, and the medullary canal is sequentially reamed with progressively larger reamers until firm endosteal engagement is established. The proximal tibia is then prepared using the metaphyseal sleeve broach attached to the appropriate trial stem. The broach is carefully impacted into the tibia until the top surface of the broach is at the desired proximal tibial resection level. If the broach is unstable or if any defect is unfilled, the procedure is repeated with consecutively larger broaches until the desired fit is achieved. Any remaining areas or voids between the periphery of the sleeve and the adjacent bone of the proximal part of the tibia are filled with morselized cancellous bone graft. The broach handle is removed, leaving the last broach in place (Figure 3). The proximal tibia is now resected using the top of the broach as a guide. The trial tibial base plate is assembled with the appropriate-sized trial metaphyseal sleeve and trial stem and inserted.
|Figure 3: The proximal tibia is resected using the top of the broach as a guide.
Joint Space Assessment
The joint space is evaluated with spacer blocks to determine the flexion-extension gaps. The balance and symmetry of both the flexion and extension gaps are established as well as whether augments are needed. With the tibia sized and the approximate joint line established, the preliminary femoral component size can be selected by evaluating the explanted component or by sizing against the cutting guide.
To decrease the flexion gap without affecting the extension gap, a larger femoral component is applied with the addition of posterior augmentation. This is particularly important where a stem extension is indicated, because the stem extension will determine the anteroposterior positioning of the component and the consequent flexion gap. The alternative is additional distal femoral resection and use of a thicker tibial insert to tighten the flexion gap. However this is generally not recommended, because considerable bone stock has already been sacrificed in the primary procedure, and it is important that additional resection of the distal femur be avoided.
To decrease the extension gap without affecting the flexion gap, the distal femur is augmented. It is important to note that this will lower the joint line, which is usually desirable because it is generally found to be elevated in knee revision patients.
The midline of the femoral trochlea is identified, and the medullary canal drilled to a depth of 3-5 cm. The medullary canal is opened sequentially with reamers of progressively larger size until firm endosteal engagement is established. Femoral metaphyseal sleeves are also available. These are used when severe bone loss is encountered in the femoral notch. Much like tibial metaphyseal sleeves, they allow for the filling of bony defects and give the femoral component a stable base for solid fixation to the metaphyseal bone. The femur is sequentially broached to the desired dimension. Care is taken to ensure that the broach remains centred in the path of the reamers. This will keep the metaphyseal sleeve in the desired anteroposterior position.
For femoral rotation, the tibial trial functions as a reference point. In a knee with competent collateral ligaments, we use a spacer block that references the tibial plateau and creates a quadrilateral flexion gap. Anterior resection is performed through the anterior slot. Posterior resection is performed through the slot designated zero or, where there is posterior condyle deficiency, the appropriate 4-mm or 8-mm slot is used to accommodate posterior augments. In most cases, little if any bone is removed from the distal femur, because the joint line is effectively elevated with the removal of the primary femoral component. Each condyle is cut only to the level required to establish a viable surface, with augmentation used to correct imbalance.
Final femoral preparation involves notch and chamfer resection. Where augmentation is planned, the appropriate augment buttons are inserted into their receptacles on the finishing guide. The length of the intercondylar box differs for the Sigma stabilised and TC3 femoral components and this needs to be taken into consideration when making the notch cut. Both are indicated on the anterior surface of the guide.
The trial femoral component is now assembled and inserted. Once the appropriate thickness polyethylene trial insert is in place, the knee is taken through a range of motion to verify function and stability. We do not routinely replace or revise the patella unless there is established or suspected infection or if it is obviously loose. At this point, the trial components are removed and the final implants are inserted.
Implantation of the components
Before final implantation of the components the site is thoroughly cleansed with pulsatile lavage and dried. The final appropriate-sized trial metaphyseal sleeve is impacted in the proximal tibial. Next bone cement is prepared and applied to the proximal tibial surface or directly to the underside of the tibial tray component which has been previously attached to the stem. These are then impacted into place and any extruded cement cleared.
Attention is then turned to the femur. As with the tibia the metaphyseal sleeve component is impacted into the distal femur. Cement is then applied to the anterior, anterior chamfer and distal surfaces of the femur and the internal posterior and posterior chamfer surfaces of the femoral component. Care is taken to ensure that the medullary canal remains free of cement. The femoral component is then impacted. Once the cement is set the trial insert is removed and the definitive mobile bearing insert introduced into the implanted tibial tray. The freedom to rotate allows the implant to bring the bearing surfaces into congruent low wear contact.
|Figure 4: AP and lateral x-rays at 18 months for a patient who underwent a revision TKR using the MBT system with metaphyseal sleeves.
Our early results using the MBT system with metaphyseal sleeves are encouraging. Porous metaphyseal sleeves were implanted during twenty six revision and three primary total knee replacements in 14 men and 15 women who had an average age of 73.3 years at the time of operation. For revision surgeries the diagnosis was aseptic loosening in 24 cases (Figure 4) and mal alignment in two cases. All patients are being followed clinically and radiographically (Figure 5). Our early results in these patients have shown good clinical outcomes with no reported failures, loosening or migration.
These results are similar to those reported by others using porous tantalum cones. Tantalum has been studied and used extensively in hip revision surgery for acetabular cups and in a wide variety of implants in other areas of medicine including pacemaker electrodes, ligation clips and mesh for nerve repair. Biomechanical studies have shown a modulus of elasticity similar to that of bone and an interconnective porous nature that allows bony ingrowth. Meneghini et al examined 15 patients with severe metaphyseal tibial defects treated with metaphyseal tantalum cones at an average follow-up of 34 months. All had evidence of osseous integration. There was no evidence of loosening or migration6. A further study by Long and Scuderi reported good early outcomes with porous tantalum cones for large tibial defects at 31 months follow-up and found no failures or loosening7.
This limited clinical data suggest that these metaphyseal sleeves and cones may facilitate making revision TKA in the setting of severe bone loss a more reproducible, predictable, and successful procedure that offers biologic fixation leading to long-term implant stability and survival.
- No authors listed. National Joint Registry. http://www.njrcentre.org.uk/ (date last accessed 15 May 2009).
- No authors listed. Scottish Arthroplasty Project Report 2009. http://www.arthro.scot.nhs.uk
- Engh GA, Ammeen DJ. Classification and preoperative radiographic evaluation: knee. Orthop Clin North Am 1998;29:205.
- Nanson CJ, Fehring TK. Stem Fixation in Revision Total Knee Arthroplasty. Techniques in knee surgery. 2009;8:163-165
- Mulhall KJ, Ghamrawi HM, Engh GA, et al. Radiographic prediction of intraoperative bone loss in knee arthroplasty revision. Clin Orthop 2006;446:51-8.
- Long WL, Scuderi GR. Porous Tantalum Cones for Large Metaphyseal Tibial Defects in Revision Total Knee Arthroplasty. Journal of Arthoplasty 2009;24(7):1086-1091
- Meneghini RM, Lewallen DG, Hanssen AD. Use of porous tantalum metaphyseal cones for severe tibial bone loss during revision total knee replacement. J Bone Joint Surg. 2008;90A:1.