Managing Bone Loss in Revision Total Knee Arthroplasty
Most patients who undergo revision TKA will have some degree of bone loss. This article reviews the implants and techniques available to surgeons to help in the management of the defects.
Corbyn Nchako, BS; Christopher Travers, MD; and Charles L. Nelson, MD
Total knee arthroplasty (TKA), one of the most reliable and effective orthopaedic surgeries, is performed to relieve pain, restore range-of-motion, and improve quality of life for the patient.  More than 600,000 primary TKAs are performed annually in the US,  and more than 55,000 revision TKAs were performed in 2010.  Nearly half of those revisions – 48% – were in patients under age 65.
The younger age of primary TKA patients and the growing annual number of TKAs have contributed to the increasing number of revision cases being performed. Current forecasts predict volume increases in primary and revision TKA of 190% in the US alone by 2030. 
Approximately 80% of revision TKA patients have some degree of bone loss. [2,5,6] Additional bone loss can occur: 
- During primary implant removal
- During debridement of potentially infected or necrotic bone
- During bone preparation for the revision implants
Long-term fixation stratagies are needed in these cases and may include: [7-9]
- Cemented or cementless stem extensions
- Bone grafts or substitutes
- Metal augments
- Porous metal cones or sleeves
The Anderson Orthopaedic Research Institute Classification is the most commonly used classification system for defining femoral and tibial bone loss during revision TKA.  Unfortunately, it is not quantitative and is challenging to use in developing a useful algorithm for managing femoral and tibial bone loss during revision TKA. The authors of this article believe there is a need for a new quantitative classification system based on degree and location of bone loss measurements of the femur and tibia, their associated landmarks, with relevant treatment algorithms.
Stem Use in Revision TKA
Both cemented and press-fit stems have been used in revision TKA to improved component stability and fixation. [11-13] A femoral stem extension is recommended in cases involving complete revision of a total knee prosthesis.  A stem is generally indicated on the tibial side, as well when the proximal bony surface is compromised. The added metadiaphyseal engagement provided by stems serves to: [7,14-18]
- Decrease interface stress in compromised bone of the distal femur and/or proximal tibia
- Offload deficient bone surfaces
- Bypass bone defects
These factors contribute to increased stability of the prosthesis bone interface and of the knee joint itself.
A classification system described by Morgan-Jones et al  considers the joint surface, the metaphysis, and the diaphysis to be the first, second, and third anatomic zones of fixation, respectively. In a revision TKA, implant stability centers on achieving fixation in at least 2 of the 3 zones.  Various stem designs can provide engagement in these zones. Metaphyseal stems are usually short, narrow, and cemented into the cancellous bone, while the diaphysis-engaging stems are longer and are often press-fit.
Although the debate on the preferred method of stem fixation is ongoing, a growing body of literature supports the claim that both cemented and press-fit stems provide similar stability. The decision to use a cemented or press-fit stem is at the discretion of the surgeon; however, when press-fit stems are utilized, the stem’s length and diameter must be sufficient to ensure diaphyseal engagement.  Shorter cemented stems and longer press-fit stems have been found to provide similar stability, [13,20] with comparable long-term survivorship and rates of recurrent aseptic loosening. [21-25] In a meta-analysis, Wang et al  found no significant differences in the rates of loosening, hardware failure, or reoperation of revision TKA with cemented versus cementless stems and no indication of superiority of either type of stem fixation.
Femoral Bone Loss
The technique used to manage femoral bone loss in revision TKA is dictated by the severity of the defect. No matter which strategy is utilized, the aim must be to: [7}
- Restore an anatomic joint line
- Restore stability
- Optimize knee-joint kinematics
In cases of condylar bone loss of up to 15mm, near joint line restoration should be achieved with distal femoral metal augments of approximately 10 mm to 12 mm. This would generally leave enough posterior condylar bone to establish prosthetic rotational stability with the posterior femoral augments.
Viable options for managing larger defects, in which 15 mm to 25 mm of bone stock has been compromised and the remaining bone stock cannot provide adequate rotational implant stability, include:
- Augmentation using bulk allograft
- Impaction bone grafting
- Porous metal sleeves or cones
Bulk allografts and impaction bone grafting have also traditionally been used in cases of uncontained defects.
The benefit of allografts has been their ability to be shaped to specific defects while providing structural support and allowing some restoration of bone stock. [26-28] Recently, however, the long-term durability of these grafts has been called into question. One study by Clatworthy et al  showed only a 72% allograft survival rate at 10 years in a series involving 52 revisions. Similarly, a review by Bauman et al  found a greater than 20% rate of complications, most being directly or indirectly allograft-related, in revision TKAs utilizing structural allograft. Infection and graft resorption, with resulting instability, were the 2 most common complications.
These concerns have turned many surgeons away from allografts and toward porous metaphyseal cones and sleeves. These porous cones and sleeves make direct contact with host bone and allow osseous ingrowth to potentiate stronger fixation.  When properly fixated, porous sleeves or cones may establish a structural metaphyseal or meta-diaphyseal foundation for the stemmed knee prosthesis. This functions to improve the construct’s axial and rotational stability and offload zone 1 fixation. [18,30-34] In addition, these components remove the risk of allograft disease transmission and graft resorption.
A survivorship study of metaphyseal sleeve usage in revision TKA by Chalmers et al  found a 5-year survivorship of 96% in femoral sleeves. Long-term follow-up is still needed to determine the efficacy of these implants, but intermediate results are promising.
The femoral epicondyles are often compromised when bone loss exceeds 25 mm, therefore disrupting the collateral ligament attachment sites and introducing global instability. Addressing collateral ligament dysfunction has traditionally been a challenge. Reinsertion and grafting techniques have shown poor results and high complication rates, leaving hinged prosthesis as the mainstay of reconstruction. Rigid hinges were historically used, but their overly constrained design led to high rates of aseptic loosening as a result of the high torsional and shear stresses transferred directly to the bone-cement interfaces. Joint distraction, even during natural movement, produced additional tensile stress on the interface as well. These added forces contributed to high rates of aseptic loosening and a revision-free survival rate of only 65% at 6 years. 
As a result of these issues, the rotating hinge platform was introduced to better simulate the biomechanics of a natural knee while helping to better offload the implant bone interface. [36,37] Rodríguez-Merchán et al  showed that at a mean follow-up of 7 years, range of motion on average improved from −15° extension and 80° flexion preoperatively to −5° extension and 120° flexion at the last follow-up. At 57 months of follow-up, Farid et al  observed an implant survival of 73%, but a periprosthetic infection rate of 22%.
The rotating hinge platform can be used as a standalone prosthesis or can also be employed with a distal femoral replacement (DFR), which has traditionally been used in limb-salvaging oncologic resectioning procedures. Although the rotating hing platform allows for early weight-bearing and more aggressive rehabilitation, it also has a relatively high complication rate, the most significant complications being infection and aseptic loosening. [40,41]
A less-commonly utilized but viable technique is allograft prosthesis composite (APC), which is often preferred to DFR in younger and more active patients because it preserves bone stock and the patient’s native ligamentous attachments. Widespread use of APC has been limited by high complication rates. Several studies have shown a 5-year survivorship rate ranging from 71% to 75% and a 10-year survivorship ranging from 61% to 72%, [29,42] with deep infection again being the major complication.
Given the less-than-stellar results of these 2 options, space exists for novel approaches to femoral bone loss greater than 25 mm with resultant collateral disruption.
Tibial Bone Loss
Many of the strategies and treatment algorithms used to address tibial bone loss mirror those of femoral bone loss and, similarly, depend on the extent of the defect. Tibial bone loss above the fibular head can generally be addressed with increasing thickness of polyethylene and a stemmed prosthesis. For unicondylar bone loss extending below the fibular head, options include cement fill with or without screws (depending on size) and metal augments. These augments, available in different shapes and size, can be used to address bone loss in the medial and/or lateral compartments. This allows for a stable cut surface that can provide balance, rotational control, and restoration of the joint line. [7-9]
Alone or as an adjunct to the techniques above, the options for managing increasing bone loss and decreasing metaphyseal support include:
- Bulk allografts
- Impaction bone grafting
- Porous metal sleeves and cones
Due to complications discussed earlier, the trend away from allografts to manage femoral bone loss and toward metaphyseal cones and sleeves applies to tibial bone loss as well. Kamath et al  found that the porous tantalum tibial cone component had a revision-free survival greater than 95% at intermediate-term follow-up of 5 to 9 years. Another study by Chalmers et al  showed metaphyseal sleeve survivorship of 99.5%.
Severe bone loss extending below the tibial tubercle and patellar tendon insertion further impacts the outcome of a revision TKA due to the need to establish extensor mechanism function. Although believed to be relatively uncommon, extensor mechanism disruption severely compromises knee function, with poor results historically from attempts to restore extensor function after knee replacement surgery.
To provide a functional revision TKA, management of bone loss below the tibial tubercle mandates a strategy that restores extensor mechanism function. Traditionally, extensor mechanism allograft has served as the mainstay of treatment. In patients with severe tibial bone loss, a proximal tibial allograft with patellar tendon, patella, and quadriceps tendon attached is an option that allows for restoration of bone stock and extensor function.
A study of 20 cases by Springer et al  demonstrated that the efficacy of extensor mechanism allograft largely depends on the initial tensioning of the allograft. Early failure was seen in the 7 cases performed with lower allograft tensioning, while the 13 cases that were tightly tensioned survived. The average postoperative extensor lag in the clinical successes was 4.3. No significant difference in postoperative flexion was observed between the 2 groups.
Other authors, however, have reported more modest outcomes, even with appropriate graft tensioning. One series reviewed 50 consecutive extensor mechanism allograft reconstructions and found an estimated Kaplan Meier survivorship of only 56% at 10 years.  The most common complications of allograft reconstruction are deep infection, allograft patellar tendon rupture, and progressive graft stretching. [43-45]
A new technique in extensor mechanism reconstruction developed at Mayo Clinic uses a synthetic mesh graft to approximate and reinforce native patellar and quadriceps tendon tissue.  The knitted monofilament polypropylene graft is used to reconstruct the patellar tendon and facilitate fixation of surrounding tissue into the graft. In this technique, the mesh graft is inserted into a trough created in the in the anterior tibia. Once inserted, it is then secured in place by cement and transfixed with a screw and washer. In tibial component revision, the graft can be cemented in place between the implant and the anterior cortical rim of the proximal tibia. 
After tibial fixation is achieved, the graft is woven from deep to superficial through a portal created in the lateral retinaculum. The remaining patellar and quadriceps tendon tissues are then mobilized. While the graft is maximally tensioned, these tissues are used to cover the graft in a sandwich fashion, preventing contact with the underlying implant or overlying superficial tissue. 
Early studies of this mesh reconstruction technique have been promising. Compared with allografts, the mesh graft reduces: [43,46,47]
- The possibility of disease transmission
- The incidence of extensor lag
- The costs of the procedure
Use of the synthetic mesh graft has become the preferred technique for extensor mechanism reconstruction at our institution, based on these promising results.
As with extensive femoral bone loss, APC can be used with significant tibial bone loss to spare collateral ligament functionality. Unfortunately, APC survivorship and complication rates are even higher with tibial bone loss than with femoral bone loss. [29,42] Additional work is needed to produce a more effective design for APC due to these high complication rates.
As the number of TKAs requiring revision rapidly increases over the next 2 decades, safe and effective techniques for the management of bone loss will continue to be at a premium. In this paper, we have examined current revision practices in a revision situation with bone loss. Long-term outcome studies are needed to evaluate the ultimate durability of many of the contemporary implants and techniques described above. Although many advances in surgical management of this difficult problem have been made in recent years, there still remains a large space for future innovation and implant design.
Corbyn Nchako, BS, is from the Saint Louis University School of Medicine, St Louis, Missouri. Christopher Travers, MD, and Charles L. Nelson, MD, are from the Department of Orthopaedic Surgery, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania.
The authors have no disclosures relevant to this article.
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