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Continuous passive motion therapy after total knee arthroplasty

Rex, Colleen, MSN, BA, CRNP, AGACNP-BC

doi: 10.1097/01.NURSE.0000531010.25095.80

After total knee arthroscopy, patients may undergo continuous passive motion therapy as part of their physical rehabilitation. Consider the potential benefits and drawbacks of this controversial therapy and learn how to ensure its safety and effectiveness.

Colleen Rex is an RN in the ICU at Pennsylvania Hospital in Philadelphia, Pa.

The author has disclosed no financial relationships related to this article.



OSTEOARTHRITIS (OA) affects 30 million U.S. adults according to the CDC, making it the most common form of arthritis and the most common joint disorder.1 OA can cause pain, stiffness, and swelling, leading to disability and diminishing the patient's quality of life.1 Fourteen million people in the United States experience symptomatic knee OA.2

OA of the knee isn't curable, but patients may experience symptom relief and improvement in quality of life through total knee arthroplasty (TKA). According to the Agency for Healthcare Research and Quality, by 2030 approximately 11 million Americans will have undergone TKA, making it one of the most common elective surgical procedures in the United States.3

Recovery after TKA requires physical rehabilitation, which may include continuous passive motion (CPM) therapy.4 This article explores the potential benefits and drawbacks of CPM therapy and discusses what nurses need to know when it's prescribed for their patients.

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Inconclusive evidence

The goal of CPM therapy is to facilitate recovery from TKA to a level that allows the patient to continue an active lifestyle.5 Earlier studies identified certain benefits for CPM therapy, including reducing pain and increasing active range of motion (ROM).6 However, more recent studies examining CPM therapy have failed to reproduce these outcomes.6 (See What does the evidence say?)

Although CPM therapy has been a standard of postoperative TKA rehabilitation protocol until recently, it's gradually fallen out of favor because current evidence doesn't justify its cost. As a result, facilities are beginning to abandon CPM therapy.5

Nevertheless, many orthopedic surgeons continue to include CPM therapy in their postoperative rehabilitation protocol.7 To achieve the desired outcome and to ensure safe patient care, nurses must feel confident and comfortable when CPM is prescribed for their patients.

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What's CPM therapy?

As an adjunct to physical therapy (PT), CPM therapy involves placing the affected joint in a specially designed motorized device that continuously moves the joint passively through a set degree of ROM.8 Therapy is typically initiated immediately following surgery in an alternating frequency of application, such as 4 hours on and 4 hours off to the postoperative limb for a specified duration, progressively increasing ROM and speed daily according to patient tolerance.

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Practice points for nurses

Orthopedic nurses, other nurses who may provide care for patients with orthopedic disorders, and home healthcare nurses all need to know about CPM therapy. Here are practice points to consider when caring for patients receiving CPM therapy:

  • CPM devices are available from various manufacturers. Nurses should always consult the manufacturers' instruction manual before attempting to operate a CPM therapy device. These instructions may also be incorporated into the healthcare facility's policy and procedure.
  • Nurses should be familiar with the controls and how to apply settings. Incorrect settings may cause complications, including damage to the prosthesis, requiring additional surgery.
  • Correctly interpreting the provider's prescription is essential to safe and effective CPM therapy. Nurses should be familiar with CPM prescription terminology such as duration, flexion, extension, and speed. (See Key terms for CPM.) They also need to know how to apply the equipment correctly, educate the patient and family about CPM therapy, and evaluate the patient's response to CPM therapy.
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Nursing considerations

Immediately after surgery while patients are recovering in the postanesthesia care unit, they may have a prescription for the knee to remain flexed at a specified angle and duration. In this case, the CPM device isn't used to move the knee through a ROM. Instead, the knee will remain in a flexed position, as prescribed, at the specified angle for the specified duration; for example, “Keep knee flexed at 70° for 4 hours, then resume CPM 0° to 30° and increase 10° daily.” This type of CPM therapy may be prescribed for patients with increased bleeding during the immediate postoperative period because it provides a tourniquet effect.9

  • Nurses must correctly apply the CPM device. First, place the patient in an upright position, with the head of the bed between 30° and 45°. The patient's knee must be aligned with the hinge joint of the CPM device to maximize comfort and prevent complications.10 Confirm that sheepskin or padding is adequately protecting any skin that's exposed to the CPM device.10 The limb should be secured in the CPM at the thigh (avoiding the surgical incision) and foot area with hook-and-loop straps, allowing the limb to rest in a neutral position.10 The nurse or the physical therapist may fit the CPM machine to the patient during application as prescribed. Because CPM therapy is an adjunct to daily postoperative PT, the nurse may be applying the CPM machine more often throughout the shift than the physical therapist.

Raising the bed rail closest to the CPM device will prevent it from falling off the bed.10 The foot of the bed should be flat, and the knee control option on the bed should be locked so the lower part of the bed isn't accidently raised, displacing the CPM device and subsequently displacing the patient's new knee joint.10 Pillows should be placed at the foot of the bed in front of the footboard if needed to prevent the CPM device from sliding away from the patient.10

To assess patient tolerance, nurses should observe the patient before the therapy is applied and through at least three or four cycles of continuous passive ROM to observe for any change in pain intensity rating or sudden uncontrolled breakthrough pain. They should also perform neurovascular assessments including color, temperature, motion, and sensation, and assess for proper fit. Signs of improper fit include the machine pinching the posterior thigh or hook-and-loop straps being secured too tightly. Aside from current trends in pain control such as intrathecal and epidural analgesia, and the more recent femoral and sciatic nerve blocks, supplemental traditional analgesics such as I.V. and P.O. opioids may be administered as prescribed.11

  • Patient education is of paramount importance. Patients should know the purpose, benefits, and possible complications of therapy; the function of equipment; and how to maintain safety precautions during CPM therapy.
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Avoiding complications

Improper implementation of CPM therapy may harm the patient, delay recovery, and damage the prosthesis.10 In addition, failure to properly fit the device can create pressure points along the operative limb, leading to pressure injuries.11 Immobility during therapy can also lead to pressure injuries on the unaffected limb.11 Failure to assess skin in direct contact with the device may result in pressure and friction injuries.11

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Being prepared

As long as CPM continues to be part of the rehabilitation protocol for many patients, nurses need to be knowledgeable about how to safely use CPM therapy for patients post-TKA.

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What does the evidence say?

Recent studies have revealed that CPM therapy doesn't demonstrate any clinically or statistically significant benefits.7 A 2015 Cochrane systematic review and meta-analysis concluded that CPM therapy post-TKA provides little benefit to recovery.7

A review of the literature revealed that the outcomes of more recent studies suggest that CPM therapy doesn't offer any long-term benefits.6 The inconsistency of outcomes of early research versus current research may be attributable to the element of immobilization in control groups. The outcomes of earlier studies were based on trials in which CPM therapy was compared with control groups in which the operative knee was immobilized.5 Standard rehabilitation varies among surgeons and facilities, but today it doesn't typically include immobilization.5

Current studies compare the contemporary practice of CPM therapy in addition to PT with PT alone. Inconsistency of interventions along with the impact of variables such as individual pain tolerance, edema, variation in procedure for measuring ROM, dose (meaning the frequency and duration of CPM therapy), lack of knowledge and inconsistency in implementation among nursing staff, and the lack of standardization of outpatient treatment may also contribute to the inconclusiveness of research regarding CPM efficacy.

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Key terms for CPM9

  • Duration refers to the length of time CPM therapy should be applied. This is usually no more than 4 hours at a time.
  • Flexion refers to the predetermined arc of motion through which the knee should move in one cycle of passive motion. Flexion is typically prescribed anywhere from 0° to 90°, but on postoperative day zero it may be prescribed at 0° to 30° and increased 10° daily.
  • Extension is typically set at 0°; in other words, the leg is straight. An example of extension range on a CPM machine may be -5° to 105°.
  • Speed is how fast the CPM machine moves the joint through the arc of motion. An example of the range of speed on a CPM machine is 1° per second on the low setting and 3° per second set on the high setting.
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