Introduction
Arthrodesis (AD) of the knee has been performed since the early 1900s [
1]. Originally it was used to treat tuberculosis of the knee joint, severe osteoarthritis and even juvenile arthritis of the knee until the 1970s. The main advantages of an AD of the knee are creating knee stability and relief of pain, both enhancing mobility. However, AD of the knee has its limitations to physical and social functioning. Kim et al. [
2] mentioned that 17 out of the 30 patients with a fused knee attempted to commit suicide because of social discrimination towards their disability. Conversion of the AD to a total knee arthroplasty (TKA) is an option for patients with AD. The conversion of a fused knee to a TKA is not only a challenging surgical procedure [
3] but it is also associated with a high incidence of complications [
2,
4‐
7]. Sufficient soft tissue and an intact extensor mechanism are important factors in achieving good clinical results of a conversion. Therefore, conversion should be evaluated on a case-by-case basis [
7,
8]. It is important that patients are highly motivated and have realistic expectations about the procedure, risk of complications and postoperative results in order to be successful [
7‐
9]. Judgment of patients generally has been positive and satisfied after conversion, even after a complicated postoperative course or outcome [
3,
7,
8].
There has been discussion on whether a painless knee fusion should be converted to a potentially painful TKA with concomitant high risks for severe complications of this surgery. Some authors advocate the procedure [
5,
6,
9‐
11], while others have expressed concern over its efficacy in the presence of the high complication rate [
1,
2,
7,
12]. Thus, a systematic review and meta-analyses of the current literature was done, to examine the role of TKA after AD of the knee and assess the clinical outcome, complications, surgical technique, patient-related outcome measures and methodological quality.
Discussion
The results of our meta-analysis show that on average there was 80° (95 % CI, 67–92°) improvement of flexion during a mean follow up of 5.5 years. The postoperative improvement of flexion was only influenced by type of TKA slightly favouring semi-constrained TKA over posterior stabilised TKA. The extensor lag was on average 13 degrees. There was a clinically relevant and statistically significant increase in HSS score: 20 points (95 % CI, 11–29 points). The extension lag was a mean of 13° (95 % CI, 6–20°). Most patients were pain-free or experienced only slight pain. Most patients were satisfied. The procedure is associated with a high rate of postoperative complications; the majority of the patients (65 %) experienced at least one complication. The most frequent complication was skin necrosis, followed by arthrofibrosis, infection and early revision. Major complications, such as refusion, amputation and death, were seen in less than 5 % of all cases. Nevertheless, most patients—including those with a complicated postoperative course—preferred their mobile knee to the fused knee [
3‐
5].
Surgical technique is paramount. The skin of the knees after AD conversion to TKA is exposed to significant stretch and might not be able to tolerate the tension that is placed upon it. In order to reduce the incidence of skin necrosis, skin grafting [
3], vascularised gastrocnemius flap [
4,
5,
7] and tissue expanders [
4] have been used. Mahomed et al. [
11] was the first to mention the use of soft-tissue expanders in AD conversion to TKA. They found that the use of soft-tissue expanders helped to provide adequate soft tissue for wound closure in their patients. No problems with wound dehiscence or skin sloughing were seen and they stated that the extensor mechanism might have been better mobilised due to using the expanders aiding for excellent ROM. This was confirmed in the study carried out by Cho et al. [
6]. Adequate exposure can also be difficult as the patella is often fused to the femur requiring an osteotomy; in addition, a tibial tubercle osteotomy is often necessary. Kim et al. [
3], have used a VY quadricesplasty to lengthen the quadriceps muscle, avoid patellar tendon avulsion, facilitate exposure and improve ROM. Their rehabilitation ranged from 6 to 20 months to overcome the weakness of the prolonged quadriceps muscle [
3]. Cameron and Hu [
4] advocated against lengthening the extensor apparatus because the quadriceps is seldom adhered to the femur and a release is relatively easy. In addition, flexion improves with time and exercise [
4]. Clemens et al. [
5] preoperatively assessed the muscle strength of both quadriceps and hamstrings in three patients showing deficits of 75 % and 78 % for quadriceps and hamstrings respectively. Postoperative extension lag might, therefore, be related to overall weakness of the extensor apparatus.
Anterior ‘overstuffing’ could result in increased tension of soft tissue, difficulties for wound closure and impaired postoperative function. In order to avoid anterior overstuffing Cameron and Hu [
4] suggested placing the femoral component as posteriorly as possible and reducing the patellar thickness. In case a stemmed femoral component is used, a very thin anterior flange with a very deep trochlea is preferred [
4]. Balancing of collateral ligaments is paramount, because collateral laxity may require a constrained prosthesis. A patellar component is not always appropriate. With knees in fixed extension, a posterior cruciate ligament (PCL) sacrificing design should be anticipated in order to allow release of the PCL and maximise the flexion potential. It has been advocated to use a constrained prosthesis to substitute for absent or deficient collateral ligaments [
6,
7]. In case of collateral ligament instability, care should be taken not to use polyethylene thicker than 18 mm, as this has been shown to elevate the joint line, increase patellar instability and to complicate skin closure [
4]. However, Kim et al. [
3] showed that adequately preserved soft-tissue sleeves were able to provide stability and were able to use a relatively non-constrained posterior stabilised TKA.
We should also consider some limitations. Most studies were of low-to-moderate methodological quality: the majority of studies did not describe a well-defined primary research question or hypothesis, or missed a predefined follow-up procedure. Included studies consisted of small case series; leaving 123 knees for analysis in this study; therefore, no definitive conclusions could be demonstrated. The most recent included paper was published in 2005, dealing with patients who were operated on between 1998 and 2002. Therefore, there is a need for high methodological quality patient series using contemporary prosthesis and surgical technique. As only one of the included studies reported PROMs [
5], future studies should also include PROMs.
In conclusion, for most patients good clinical results are achieved albeit at a high risk of postoperative complications. Conversion of AD to TKA should, therefore, only be performed on a highly individualised basis with both patients and surgeons being aware of the clinical results and complications. Future studies should include PROMs and could benefit from improved methodological design.
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