Key points

Introduction

Primarily used to reduce joint stiffness after joint surgery or trauma, continuous passive motion (CPM) therapy works to counteract the pathologic stages of joint stiffness: bleeding, edema, granulation tissue, and fibrosis. This postoperative therapy has been used for a variety of orthopedic surgeries, including the management of total knee arthroplasty, fracture repair, rotator cuff repair, hand rehabilitation, and reconstruction rehabilitation of the anterior cruciate ligament. Salter pioneered the use of CPM in the 1980s after observing that the therapy stimulated articular cartilage healing and prevented complications caused by immobilization after injury in rabbit models ( Fig. 1 ). Further animal studies went on to investigate the role of CPM therapy in reducing joint stiffness after intra-articular injury.

Improved cartilage repair seen with CPM from animal studies. First index of healing: the nature of the reparative tissue at 3 weeks in the 36 defects in each of the 3 series in adult animals. The bars depict the percentages of the 36 defects in each series that showed predominantly hyaline cartilage, incompletely differentiated mesenchymal tissue, and fibrous tissue. The nature of the reparative tissue in the defects treated with CPM (series III) is superior to that after either immobilization (series I) or intermittent active motion (series II).

( From Salter RB, Simmonds D, Malcolm B, et al. The biologic effect of continuous passive motion on the healing of full-thickness defects in articular cartilage. J Bone Joint Surg Am 1980;62:1246; with permission.)

However, whereas studies conducted on animal models show a significant benefit in CPM use after injury, studies performed in a clinical setting show more conflicting results. The many variations in clinical CPM protocols could be partially to blame for this lack of agreement, with no standard method of use dictating the number of degrees per day that the machine should advance or the number of hours per day that the treatment should last. Despite this lack of conclusive evidence showing definitive benefits when used clinically, CPM therapy has become standard practice in many centers for postoperative treatment of many joint injuries. However, CPM therapy remains a highly debated treatment, with some recent studies highlighting the disadvantages of the treatment, such as the need for the patient to stay in bed, the increased costs of maintaining and operating the units, and the extra technical support that patients require from their nurses. With the potential of CPM to facilitate faster recovery, shorten patients’ length of stay, and, as a result, reduce costs, many hospitals could benefit from a definitive verdict on the effectiveness of CPM therapy.

Much of the clinical research focuses on the efficacy of this treatment in increasing range of motion and decreasing hospitalization time and postoperative complications after total knee arthroplasty when compared with a regimen focused on physical therapy alone. However, little research exists on the use of CPM for the management of articular fractures. Many articular fractures, such as tibial plateau fractures, can develop stiffness as a sequela. Recovery from this fracture, as an example, is also often further complicated by significant soft tissue injury and can involve collateral ligaments and the anterior and posterior cruciate ligaments. In addition, the significant amount of bleeding associated with the soft tissue injury and fracture of the proximal tibial metaphysis can lead to compartment syndrome, and postoperative complications such as deep vein thrombosis can develop. Given the nature of these possible complications and the proposed benefits of CPM, which include the potential to decrease hemathrosis and decrease the incidence of deep vein thrombosis in patients with trauma, CPM therapy has may offer many advantages postoperatively. However, our understanding of the efficacy of CPM in the management of articular fractures is not well understood, because few studies have specifically examined CPM in this setting, although its use in patients with total knee arthroplasty has been examined.

In this article, the rationale and basic science evidence for CPM in articular injuries are reviewed and also the clinical evidence in the postoperative treatment of intra-articular fractures.

Introduction

Primarily used to reduce joint stiffness after joint surgery or trauma, continuous passive motion (CPM) therapy works to counteract the pathologic stages of joint stiffness: bleeding, edema, granulation tissue, and fibrosis. This postoperative therapy has been used for a variety of orthopedic surgeries, including the management of total knee arthroplasty, fracture repair, rotator cuff repair, hand rehabilitation, and reconstruction rehabilitation of the anterior cruciate ligament. Salter pioneered the use of CPM in the 1980s after observing that the therapy stimulated articular cartilage healing and prevented complications caused by immobilization after injury in rabbit models ( Fig. 1 ). Further animal studies went on to investigate the role of CPM therapy in reducing joint stiffness after intra-articular injury.

Improved cartilage repair seen with CPM from animal studies. First index of healing: the nature of the reparative tissue at 3 weeks in the 36 defects in each of the 3 series in adult animals. The bars depict the percentages of the 36 defects in each series that showed predominantly hyaline cartilage, incompletely differentiated mesenchymal tissue, and fibrous tissue. The nature of the reparative tissue in the defects treated with CPM (series III) is superior to that after either immobilization (series I) or intermittent active motion (series II).

( From Salter RB, Simmonds D, Malcolm B, et al. The biologic effect of continuous passive motion on the healing of full-thickness defects in articular cartilage. J Bone Joint Surg Am 1980;62:1246; with permission.)

However, whereas studies conducted on animal models show a significant benefit in CPM use after injury, studies performed in a clinical setting show more conflicting results. The many variations in clinical CPM protocols could be partially to blame for this lack of agreement, with no standard method of use dictating the number of degrees per day that the machine should advance or the number of hours per day that the treatment should last. Despite this lack of conclusive evidence showing definitive benefits when used clinically, CPM therapy has become standard practice in many centers for postoperative treatment of many joint injuries. However, CPM therapy remains a highly debated treatment, with some recent studies highlighting the disadvantages of the treatment, such as the need for the patient to stay in bed, the increased costs of maintaining and operating the units, and the extra technical support that patients require from their nurses. With the potential of CPM to facilitate faster recovery, shorten patients’ length of stay, and, as a result, reduce costs, many hospitals could benefit from a definitive verdict on the effectiveness of CPM therapy.

Much of the clinical research focuses on the efficacy of this treatment in increasing range of motion and decreasing hospitalization time and postoperative complications after total knee arthroplasty when compared with a regimen focused on physical therapy alone. However, little research exists on the use of CPM for the management of articular fractures. Many articular fractures, such as tibial plateau fractures, can develop stiffness as a sequela. Recovery from this fracture, as an example, is also often further complicated by significant soft tissue injury and can involve collateral ligaments and the anterior and posterior cruciate ligaments. In addition, the significant amount of bleeding associated with the soft tissue injury and fracture of the proximal tibial metaphysis can lead to compartment syndrome, and postoperative complications such as deep vein thrombosis can develop. Given the nature of these possible complications and the proposed benefits of CPM, which include the potential to decrease hemathrosis and decrease the incidence of deep vein thrombosis in patients with trauma, CPM therapy has may offer many advantages postoperatively. However, our understanding of the efficacy of CPM in the management of articular fractures is not well understood, because few studies have specifically examined CPM in this setting, although its use in patients with total knee arthroplasty has been examined.

In this article, the rationale and basic science evidence for CPM in articular injuries are reviewed and also the clinical evidence in the postoperative treatment of intra-articular fractures.

CPM therapy: investigating potential benefits

The historical progression to the development of CPM started off with early research performed through the 1950s, 1960s, and 1970s, which demonstrated the effects of immobilization compared with joint motion on articular cartilage. These early studies provided evidence of the harmful effects of immobilization, which caused deterioration and articular cartilage loss in animal models. Fibrocartilage replaced the articular cartilage, and adhesions developed after immobilization; after 30 days of immobilization, the cartilage damage could not be reversed like it could be with changes seen in soft tissue. However, this damage could be prevented if immobilization was limited and early exercise was emphasized.

Salter pioneered the use of CPM through his early work starting in the 1970s. He and his colleagues conducted numerous studies on rabbit models and specifically looked at CPM therapy in improving the outcomes in synovial joint injuries. Salter compared CPM therapy with immobilization in his rabbit models, starting CPM immediately after surgery and continuing nonstop for 1 to 4 weeks. He found that the new therapy stimulated healing of the articular cartilage and led to faster and better healing when compared with both immobilization and limited active motion. In looking specifically at intra-articular fractures, CPM therapy stimulated articular cartilage growth and therefore was protective against degenerative arthritis development and resulted in better surgical wound healing. In his 1984 publication Salter summarized his findings and presented an early report on the clinical applications of CPM. In this retrospective study, he observed the effects of CPM for various joint injuries of the hip, knee, ankle, elbow, and finger. Salter’s early research summarized in this case study supports the use of CPM therapy in preventing joint stiffness and facilitating healing, specifically for articular cartilage. Early success with rabbit models in the treatment of full-thickness articular cartilage defects, intra-articular fractures, acute septic arthritis, reconstruction of the medial collateral ligament, and lacerations of tendons encouraged the use of CPM therapy in clinical applications. The 9 cases that Salter reviewed used CPM therapy for a variety of injuries (2 intra-articular femur fractures, 1 patellar dislocation, 2 elbow fractures, 1 acetabular fracture, 1 intra-articular finger fracture, 1 hip infection, and 1 case of arthrofibrosis). This is clearly a heterogeneous group of cases. The protocol for CPM therapy followed by and recommended by these case studies indicates immediate postoperative use, starting in recovery and continuing without prolonged interruption for 1 week at 1 cycle per 45 seconds. Success in the clinical setting mimicked the early experimental success with patients treated with the CPM therapy, reporting that they tolerated the treatment well and maintained the increased range of motion achieved through their respective surgical procedures. In addition, the case studies showed no CPM-related complications, periods of prolonged hospitalization, or increase in patient pain or discomfort.

Basic science evidence

Frequency and treatment parameters: basic science and clinical evidence

As mentioned earlier, CPM is used frequently by clinicians, but there are few guidelines for the timing of treatment, frequency, duration, and other treatment parameters. Studies by Gebhard and colleagues and Shimizu and colleagues further showed the benefits of CPM therapy and also set forth more specific parameters of use. Both studies used animal models to find the ideal number of hours per day needed to obtain the benefits of CPM therapy. Another study by Takai and colleagues looked at the effect of the frequency of the CPM machine cycles on the healing of tendons. This study indicated that the frequency might allow for a shorter duration of use with the same benefits.

Investigating duration of treatment, Gebhard and colleagues looked specifically at joint stiffness, muscle mass, bone density, and regional swelling after intra-articular injury, using rabbit models. Thirty rabbits received an intra-articular injury by a tibial pin drilled into their ankle joints. The rabbits were then divided into 5 groups to receive 4, 8, 12, 16, or 24 hours of CPM each day on 1 injured ankle and immobilization on the other ankle. When not undergoing CPM therapy, the rabbits were immobilized. After 3 weeks, the rabbits were evaluated. In looking at each of the parameters measured, Gebhard and colleagues found that only the rabbits treated with either 16 or 24 hours of CPM therapy had any benefits in reducing joint stiffness. Rabbits that received the shorter duration CPM therapy showed a worsening in mobility, with the CPM-treated limbs as much as 4 times stiffer than immobilized limbs. In terms of swelling, the 24-hour group was the only to show any benefit, although the decrease was not significant. All of the CPM groups increased in muscle mass, being 13% greater than the immobilized limb. However, bone density went against the previous trend, with longer CPM duration having more benefits, and an increase in bone density was observed only in those treated with 12 hours or less of CPM therapy. Bone density data showed a statistically significant inverse relationship between duration and bone density; those treated with 12, 8, and 4 hours of CPM per day had progressively more bone density than those with immobilization or 16 and 24 hours CPM per day. Through their experiments with animal models, Gebhard and colleagues showed the differing effects of CPM therapy on different tissue types and recommended that the therapy be used for at least 16 hours per day to prevent stiffness, reduce swelling, and increase muscle mass, without having detrimental effects on bone density.

Shimizu and colleagues also focused on the dose-response relationship of CPM therapy. The study again used rabbit models, and in both knees of all 34 rabbits, the investigators exposed the knee joint and dislocated the patella as well as creating holes in the articular bone of the femur. Postoperatively, the rabbit subjects were divided into groups based on the number of hours per day that they would receive CPM treatment. All CPM machines were set at the same arc and cycle duration and the same immobilization cast, set at 90° flexion, was used. Ten rabbits received CPM therapy 24 hours a day; 6 rabbits received CPM for 8 hours a day and immobilization for the remaining time on 1 joint and CPM for 2 hours a day with immobilization on the other; 7 rabbits remained immobilized for the full 2 weeks; 9 rabbits were allowed normal cage activity for the full duration; and 5 rabbit knees received immobilization for 1 week followed by 1 week of 24-hour-a-day CPM therapy. After treatment, the rabbits were allowed normal cage activity for an additional 5 weeks before being evaluated. Shimizu and his colleagues examined mobility, histologic features, and the extent of cartilage repair. Although no significant difference was found in passive mobility, visual and histologic analysis of the joints treated with CPM for 24 hours per day and for 8 hours a day showed better repair and healing compared with the immobilized and cage activity groups. In addition, CPM conducted after 1 week of immobilization did not overcome the initial harm caused by immobilization. The findings led the group to recommend that CPM therapy should be started as soon as possible and that the most favorable results are achieved when CPM is performed for 8 to 24 hours a day, although brief periods of immobilization left no ill effects.

Takai and colleagues suggested that the cycles per minute of the CPM machine might allow for shorter durations of use. In their study, they used dogs as the animal model and after flexor tendon injury and repair, the dogs were divided into 2 treatment groups. One group received CPM therapy for 5 minutes per day at 12 cycles per minute, whereas the second group received the same therapy for 60 minutes a day at 1 cycle per minute. These parameters resulted in the same number of cycles per day, but at different frequencies. After harvesting the tendons, the gliding function and strength of the tendons were evaluated at 3 weeks and 6 weeks. Although the function of the tendons was the same for both groups, the tendon strength of the higher-frequency group was significantly greater. Therefore, although duration of CPM therapy is an important variable in the effectiveness of the therapy, the frequency of cycles might have an even greater effect on outcome.

Another important parameter that, like the others discussed earlier, remains unstandardized is the number of days that the patient must use the CPM machine to obtain any benefits. Several clinical studies have looked at this variable. One study determined that 3 days of CPM therapy was sufficient after looking at effects of the therapy on 2 groups of patients. The first group experienced postoperative knee or elbow stiffness that existed for some time before therapy, whereas the second group used CPM therapy immediately after the injury. After only 3 days of therapy, the first group saw significant improvements in range of motion, which was maintained on follow-up, whereas the second group regained their preinjury range of motion with the reduced CPM therapy duration as well. Other studies looked at patients after total knee arthroplasty. In a study by Bennett and colleagues, an early-flexion CPM group started at a greater degree of flexion in recovery and continued the treatment for 7 days, comparing the outcome with a standard CPM group and a control with no CPM therapy. Overall, the early-flexion group showed significantly greater range of motion early on, but the groups showed similar results after 1 year of follow-up. Similarly, other studies comparing the number of hours per day dedicated to CPM therapy found no significant difference in the range of motion of the patients with total knee arthroplasty. Overall, the literature suggests that no consensus has been reached on the optimum number of hours per days and the number of days that the CPM therapy should be administered.