Summary of main results

CPM following TKA is primarily advocated for its proposed benefits on knee ROM, particularly knee flexion. Knee flexion ROM is important for mobility tasks such as walking, transferring and performing activities of daily living (Rowe 2000). It is also important for activities such as squatting and kneeling, which are activities of daily living for many cultures (Hemmerich 2006). In interpreting the results of this review, it is important to consider how much additional knee flexion ROM is required to justify the use of CPM. Few people would claim that an added benefit of less than 5 degrees is functionally important, and most people would probably agree that considerably more than 5 degrees is required to justify the added time, cost and inconvenience of CPM. This being the case, the findings of this systematic review provide moderate‐quality evidence to indicate that CPM does not have clinically important short‐term effects on active knee flexion ROM (see summary of findings Table for the main comparison and Analysis 1.1). The short‐term effects on passive knee flexion, passive knee extension and active knee extension are similar with mean effects of 1 to 2 degrees. All estimates of the short‐term effects of CPM on knee ROM are estimated with less than 5 degrees of uncertainty (as reflected in the 95% CI). The medium‐ and long‐term effects of CPM on active knee flexion ROM, passive knee flexion ROM, active knee extension ROM or passive knee extension ROM are similar to the short‐term effects ‐ all have mean effects less than 3 degrees. Importantly, the upper 95% CIs of all but one of the eight estimates of the medium‐ and long‐term effects are less than 5 degrees. The exception is the long‐term effects on active knee flexion ROM, but this estimate was only based on three trials of 132 participants and the estimate is imprecise (95% CI ‐1 to 7 degrees). Taken together, this review provides moderate‐quality evidence that CPM does not have sufficient effect on knee ROM to justify its widespread use.

Some people develop very stiff knees following TKA, irrespective of whether they receive CPM. One way to manage this problem is to manipulate the knee under anaesthesia. There is very low‐quality evidence to suggest that CPM reduces the risk of manipulation. The reported effect is large in relative terms (RR 0.34) but the incidence proportion of manipulations following TKA is low (i.e. 7%) with only 25 manipulations reported in the eight trials involving 581 participants. An RR of 0.34 with this incidence proportion corresponds to an absolute reduction in risk of manipulation under anaesthesia of 4% (see summary of findings Table for the main comparison). This means that one manipulation will be prevented in every 21 people provided with CPM (95% CI 16 to 126). However, the estimate of RR is imprecise (95% CI 0.13 to 0.89) and does not rule out the possibility of a trivially small effect (i.e. one manipulation prevented in every 126 people provided CPM). In addition, these results are based on just eight trials and it is not clear in any of the included trials whether the clinicians making decisions about the need for manipulation were blinded to allocation. Interestingly, while knee stiffness is the most common indication for manipulation under anaesthesia, the results from this systematic review indicate there was little difference in knee ROM of people who did and did not receive CPM. Therefore, the findings about risk of manipulation need to be interpreted with caution (see summary of findings Table for the main comparison and Analysis 1.14).

One hundred and seventy‐eight adverse events were reported in 16 trials comprising 1040 participants. The types of adverse events reported were delayed healing, haemarthrosis, falls, deep venous thromboses, wound infections, pulmonary emboli and a patellar rupture. Some of the adverse events were unlikely to be related to CPM but nonetheless were included in the analyses because they would not bias the findings. The point estimate provides low‐quality evidence to indicate a decrease in the risk of adverse events with CPM (RR 0.92; 95% CI 0.63 to 1.33); however, the associated 95% CI includes an important increase and decrease in the risk of adverse events with CPM. It is, therefore, unclear whether CPM affects the risk of adverse events. The effect of CPM on participants' global assessment of treatment effectiveness was also unclear. The three trials that included this outcome provided contradictory results and the data were too heterogeneous to pool.

CPM clearly does not have clinically meaningful short‐, medium‐ or long‐term effects on pain (MDs range from ‐0.4 to 0.3 points on a 10‐point scale; 95% CI range from ‐0.8 to 0.9) although the medium‐ and long‐term effects are only based on three and one trials, respectively, and, therefore, need to be interpreted with caution. The short‐, medium‐ and long‐term effects of CPM on length of hospital stay, swelling and quadriceps strength are more difficult to interpret. While none of the meta‐analyses on these outcomes were statistically significant, it is not clear if they rule out clinically meaningful effects. Resolution of this issue requires clarification of what constitutes a clinically meaningful effect. For example, if a reduction in length of hospital stay of 2 days is considered clinically meaningful then the results of this systematic review clearly indicate that CPM does not have clinically meaningful effects (MD ‐0.4 days; 95% CI ‐1.0 to 0.2). However, if a reduction in length of hospital stay of 1 day is considered clinically meaningful then the data do not rule out the possibility of a clinically meaningful effect.

Perhaps the most important outcome is function because it reflects the implications of ROM, strength, swelling and pain on activities of daily living. However, function was not routinely included as an outcome measure and when it was, many different scales were used. The best estimate of the effect of CPM on function comes from trials that examined the medium‐term effect. These trials provided moderate‐quality evidence to suggest that CPM does not have clinically important effects on function unless a relative percent change in function of 5% is considered as worthwhile (relative percent change ‐4%; 95% CI ‐12 to 5; see summary of findings Table for the main comparison). The precision of this estimate is surprisingly good given the multidimensional nature of the different measures used to measure function. For example, The Hospital and Knee Score used in some trials not only measures the ability of patients to walk, transfer and climb stairs but also measures knee ROM, pain, strength and instability. Future trials would benefit from some agreement on the most appropriate way to measure function in this group of patients.

Quality of life was only measured in two trials (Bennett 2005; Maniar 2012), yet together these trials provide moderate‐quality evidence to indicate that CPM does not have clinically important effects. The medium‐term effect was very small and the estimate surprisingly precise (MD of 1 point on a 0‐ to 100‐point scale; 95% CI ‐3 to 4). This estimate does not rule out the possibility of a 4/100 point improvement in quality of life with CPM but most would consider this too small to justify its use. However, more trials are required to verify these findings.

Overall completeness and applicability of evidence

Many different protocols were used to administer CPM. For example, in some trials CPM was started immediately after the TKA operation whereas in other trials it was started days later. The CPM settings were also different between trials. In some trials, the settings were dictated by a protocol whereas in other trials they were determined by patient comfort or clinician discretion. All these variables may have influenced the observed effects of CPM. A meta‐regression was used to explore the possibility that total CPM time influences passive knee flexion ROM. Passive knee flexion ROM was selected because this was the most commonly measured outcome. The meta‐regression indicated that there was little or no effect of CPM duration on short‐term passive knee flexion ROM. That is, trials that applied CPM for 24 hours a day over an extended time period did not report systematically more passive knee flexion ROM than those that only applied CPM for a few hours over one or two days.

Protocols for co‐interventions were also highly variable. In a subset of four trials, control participants also received additional knee exercises. A secondary analysis explored the effect of CPM in this subset of trials. The purpose was to compare the effectiveness of CPM and knee exercises. This analysis was inconclusive because of the small number of trials. Nonetheless, it is unlikely that CPM would be more effective than knee exercises because the primary analysis indicated CPM is no more effective than usual care, with or without additional knee exercises.

There was usually a high degree of consistency (low between‐study heterogeneity) in estimates of effects of pooled findings. Heterogeneity was only apparent in a small number of comparisons, typically where SMDs were used and when total sample sizes were small. The heterogeneity could have been due to any number of factors but was most likely due to the use of different tools to measure the same construct. This was particularly problematic for function, which was measured with outcomes as diverse as self reporting questionnaires and timed walking tests.

Interestingly, the applicability of the evidence about CPM is changing. CPM may no longer be a viable or appropriate treatment option, regardless of findings, because patients are now commonly discharged within a few days of surgery and often mobilised on the same day as surgery.

Quality of the evidence

The methodological quality of trials was variable (see Figure 2). Only 10 trials clearly blinded assessors. Not surprisingly, given the nature of the intervention, no trial blinded participants or therapists. Failure to blind assessors, participants and therapists exposes the trials to performance and detection biases. The failure to blind participants is potentially more of problem for outcomes that rely on self report (such as pain) than more objective measures (such as passive knee ROM). Only eight of the 24 trials concealed allocation and nearly all trials were selective in their reporting of data. These potential sources of bias led to a downgrading of the quality of evidence for all outcomes reported in the summary of findings Table for the main comparison on the 'Risk of bias' criterion. Pain, manipulation under anaesthesia and participants' global assessment of treatment effectiveness were further downgraded because the results across trials for these outcomes were inconsistent. Imprecision of the point estimates were considered a serious problem for two of the primary outcomes, namely participants' global assessment of treatment effectiveness and manipulation under anaesthesia. Consequently, the findings for active knee flexion ROM, function and quality of life are based on moderate‐quality evidence, the findings for pain and risk of adverse events are based on low‐quality evidence, and the findings for participants' global assessment of treatment effectiveness and manipulation under anaesthesia are based on very low‐quality evidence. These limitations notwithstanding, the main findings are probably robust because bias tends to inflate estimates of effects (Gluud 2006; Savovic 2012), and most estimates of treatment effects in this systematic review were very small.

Potential biases in the review process

The main potential source of bias in the review process arises from failure to identify all relevant trials. This may have occurred, particularly if trials were unpublished. However, retrieval bias generally tends to inflate estimates of effects (Dickersin 1993; Egger 1998), but this review reports small effects of CPM on most outcomes.

Agreements and disagreements with other studies or reviews

The original version of this systematic review was done in 2003. It was updated in 2010 and then again for this review in 2013. The main difference in conclusions between the original and the 2010 version related to length of hospital stay. In 2003, it was concluded that CPM reduces length of hospital stay. In the 2010 systematic review, it was concluded that CPM does not reduce length of hospital stay. However, in this present review it was concluded that the effects of CPM on length of hospital stay are inconclusive.

It is also worth noting that the original review performed in 2003 suggested that the effect of CPM on knee ROM may be dose dependent. Dose dependence was tested in the 2010 review and then again in this review by using a meta‐regression to examine effects of mean total CPM time (hours) on passive knee flexion ROM in the short term. The results indicated that the response to CPM is not dose dependent.

Our findings are broadly consistent with the findings of similar systematic reviews (Grella 2008; Lenssen 2003b; van Dijk 2007; Viswanathan 2010). The only discrepancy is with the conclusions of Lenssen et al (Lenssen 2003b). These authors concluded that CPM had a short‐term effect on knee flexion ROM (MD 8 degrees; 95% CI ‐2 to 18) despite the considerable uncertainty around the estimate. The difference between our estimate and that of Lenssen 2003b was probably due to differences in the trials included in the two meta‐analyses. Our pooled estimate (MD 2 degrees, 95% CI 0 to 4) was based on 11 trials (697 participants) and the estimate of Lenssen 2003b was based of five trials (317 participants). There was only one trial in common in the two meta‐analyses due to differences in the inclusion criteria and year of the reviews.

Other systematic reviews have investigated the effect of CPM following knee cartilage defect surgery (Fazalare 2010), shoulder rotator cuff repair (Du Plessis 2011), and anterior cruciate ligament reconstruction (Lobb 2012; Smith 2007). One review reported no evidence of benefit of CPM (Lobb 2012), two concluded that there was insufficient evidence (Fazalare 2010; Smith 2007), and one reported two clinical trials that demonstrated a treatment effect on ROM but this conclusion was not clearly supported from the findings of the two cited trials (Du Plessis 2011). While it is possible that the effects of CPM are different for various conditions, this would seem unlikely. Therefore, the results of the systematic reviews following different types of surgery lend further weight to the results of this systematic review indicating little if any benefit of CPM on at least knee ROM following TKA.