Take-Home Points
- Ensure that relative rest and activity modification allow the knee to stay within the available “envelope of function” of the joint.
- Careful physical examination is imperative to assess strength, flexibility, and altered movement patterns, which in many cases are all part of the etiology of AKP.
- Patience and perseverance are paramount. Patients need to clearly understand the goals of rehabilitation as well as the concepts related to “envelope of function” so they can continue to keep themselves within this envelope. This education is crucial to their success.
- Only once a patient has been brought into the pain-free functional envelope can rehabilitation be redirected to expanding the envelope toward the patient’s particular goals.
- Quantity does not equal quality. To create an appropriate care plan, the physician must assess the adequacy of the patient’s rehabilitation thus far—ask specific questions about the types of exercises the patient is doing in physical therapy and quickly assess strength with a few simple in-office tests.
Anterior knee pain (AKP) is a common presentation. Although the exact etiology and nature of AKP continue to be poorly understood, overuse principles can be useful in directing treatment. In overuse injury, repetitive submaximal or subclinical trauma results in macroscopic trauma, microscopic trauma, or both. The structural tissue unit is damaged or its clinical responsiveness is exceeded, which can lead to pain or movement dysfunction. Overuse injuries commonly have an endogenous source, mechanical circumstances in which the musculoskeletal tissue is subjected to more tensile force or stress than the tissue can tolerate. The approach to treatment and rehabilitation of AKP is best facilitated with a thorough understanding of the concept of tissue homeostasis and the “envelope of function.”
Although the cause of AKP is multifactorial, the contributions of muscle strength deficits, diminished neuromuscular control, and altered muscle firing patterns to the development and severity of AKP are well established.1-5 The hallmark of nonoperative management of AKP is physiotherapy that re-establishes strength, neuromuscular control, muscle activation, and optimal biomechanics during daily activities, advancing to graded levels of sporting activities.
The purpose of this paper is to discuss the factors associated with the diminished neuromuscular control observed in AKP and to review appropriate rehabilitation concepts for patients with AKP. Practical tools are provided to aid the surgeon to identify neuromuscular deficits in the clinic setting, along with assessing the adequacy of prior therapy and the need for further rehabilitation.
Common Neuromuscular Deficits in AKP
Weakness of the knee extensor muscles has long been implicated as the main issue in AKP, and therefore the focus of rehabilitation has been on muscle strengthening, especially of the vastus medialis obliquus. Research has found that knee extensor weakness is not only a characteristic of patients with AKP but a risk factor for developing AKP.4 Restoration of knee extensor strength and function is essential for recovery.6 Another issue in AKP may be incorrect firing of the knee extensor muscles. Altered vastus medialis obliquus response time and a motor control deficit of the quadriceps musculature have been demonstrated.7,8 Restoration of knee extensor strength, though important, is too often the sole focus of some rehabilitation programs.
Hip muscle weakness has also been implicated as an important component of AKP.9-12 Impaired gluteal muscle function can lead to increased hip joint adduction and internal rotation during activities such as stair climbing, squatting, and sports.9,10,13 In a systematic review, Meira and Brumitt12 concluded that hip strength and position are linked to AKP and that patients with AKP present with a common deficit once symptomatic. The dysfunction in neuromuscular control in AKP may also stem from disordered firing sequences in the muscles. A systematic review of hip electromyographic studies found moderate to strong evidence that gluteus medius muscle activity is delayed and of shorter duration during stair ascent and descent in patients with AKP.11 The study also found some evidence that this activity is delayed and of shorter duration during running and that gluteus maximus muscle activity is increased during stair descent. The authors recommended that interventions focused on correcting these deficits—such as hip strengthening, biofeedback, and gait retraining—should be included in AKP treatment and research.
In recent AKP research, the core, including hip and abdominal muscles, demonstrated decreased strength and altered recruitment patterns during functional movement.14,15 The authors recommended including core strengthening and core stability exercises in AKP management. In combination, these knee extensor, hip, and core strength deficits in patients with AKP lead to altered movement patterns during functional activities and may in turn exacerbate symptoms. Addressing both the strength deficits and the recruitment patterns of these core and lower extremity muscles is essential for optimizing rehabilitation and limiting recurrence of AKP symptoms.
Stretching to improve muscle tendon length is another component of AKP treatment. Reduced quadriceps muscle length has been implicated as a cause of AKP and is a common finding in symptomatic patients.16 In addition, a recent randomized controlled trial found decreased hip flexibility in patients with AKP.17 It is important to assess the flexibility of the gastrocnemius, soleus, quadriceps, and hamstrings muscles and the iliotibial band, as well as the hip flexors, extensors, and rotators, so that rehabilitation can be designed to address any specific deficits in range of motion (ROM).16-23 In patients with AKP, it is also important to address muscle tendon length deficits and strengthening simultaneously to avoid exceeding the available envelope of function. Gaining full ROM at joints can facilitate increasing strength gains24 and potentially improve the synergy of muscle contractions during functional activities.