Introduction

Participation in physical activity is at the forefront of international health promotion agendas, and there is increasing encouragement for the maintenance of participation in sports and exercise activities throughout an individual’s life span.¹ Moderate recreational physical exercise is associated with a decrease in the risk of knee osteoarthritis.² However, participation in high-impact sports can increase the risk of developing articular cartilage lesions,^3,4 and athletes are at a higher risk of developing knee osteoarthritis.⁵ There is increasing evidence that excessive stress on a joint with an articular cartilage lesion may accelerate further degenerative changes⁶ and that, if unaddressed, small cartilage defects progress to osteoarthritis.⁷ Messner and Maletius established that 75% of young athletes demonstrated radiographic joint space reduction an average of 14 years after sustaining chondral damage and returning to their preinjury sport levels.⁸ Consequently, it is not surprising that individuals are seeking out elective surgical procedures more frequently in order to address cartilage defects with the aim of maintaining or regaining their ability to participate in sports and exercise activities.

The ultimate goal for surgical intervention on chondral defects is the restoration of the joint surface to restore “normal” hyaline articular cartilage. Since the late 1980s, orthopaedic surgeons and tissue engineers have immersed themselves in this quest, resulting in the development of a new range of surgical procedures to restore the structure of normal articular cartilage.⁹ The treatment of articular cartilage defects has undergone a rapid and exciting evolution in recent years, most notably in the field of advanced cell-based orthobiological technologies. However, although some of these surgical interventions have demonstrated great promise, the new tissue in its current state does not precisely replicate natural articular cartilage in terms of form and function. Consequently, these interventions can only be considered reparative rather than the desired restorative.

Postoperative rehabilitation protocols have been developed for articular cartilage implantation, but they continue to be invalidated in a statistical manner. At present, the evidence base for autologous chondrocyte implantation (ACI) rehabilitation is in its infancy. Prior experience with related surgical procedures that are quickly evolving has shown that when the evidence base for rehabilitation is limited, fears of graft failure are paramount. This concern, in conjunction with the relative minority of therapists with experience treating ACI patients, is likely to be reflected in an overcautious approach to ACI rehabilitation at the present time.

To maximize the benefits of ACI surgery, it is essential for patients to be well informed and educated in order for them to comply to a specific rehabilitation program.^9–11 Patient education, the management of patient expectations, and clear goal setting are indispensable within ACI rehabilitation. These values are reliant on a collaborative environment, with thorough communication between the surgeon, therapist, and patient. In addition, patient selection and the patients’ respective characteristics can affect the functional outcome in an inordinate way. deWindt et al. published a study examining the prognostic factors related to cartilage implantation and found that in lesions smaller than 3 cm², defect location and defect age were statistically linked to better outcome scores on the knee injury and osteoarthritis outcome score (KOOS) at three years after ACI surgery.¹² We cannot underestimate the notion of patient selection and the characteristics of the defect itself.

The two primary goals for an ACI rehabilitation program are as follows:

The three main components of the rehabilitation program are as follows:

The repair site is at its most vulnerable during the first 3 months after ACI. At this time, it is important to avoid impact as well as excessive loading and shearing forces.

There is a consensus of opinion that weight bearing and ROM should be restricted in early rehabilitation, but there is considerable variation across cartilage repair centers as to the extent and duration of these restrictions.

Rehabilitation following an ACI procedure typically begins the day after the surgery, beginning with continuous passive motion (CPM). The frequency, duration, and ROM will depend on the location and size of the lesion. Active and passive motion is also utilized to facilitate the integration of the graft into the surrounding articular cartilage and subchondral bone. CPM is clinically recommended for 2 to 8 hours per day (depending on the site of the lesion) at one cycle per minute during the early phase of rehabilitation.

This provides a cyclical compression/decompression to allow mechanical stimulation of the graft to promote chondrocyte growth. Additional goals include alleviating pain and edema, addressing soft tissue adaptive changes, restoring muscle strength and function, and gradually including progressive resistive exercise to allow a return to the prior level of function. The insight gained from a preoperative evaluation will be invaluable to the physical therapist when designing the postoperative rehabilitation protocol. Having the knowledge of the underlying biomechanical deficits present before surgery, obtaining the perioperative report to delineate the nature and location of the articular cartilage repair, and having an open dialogue with the respective surgeon will ultimately improve the overall functional outcome of both the surgical and rehabilitation interventions imparted to the patient.

An understanding of applied clinical biomechanics and an appreciation of the forces and loads that will be exerted on the graft are essential in the design of an ACI rehabilitation program.

The contact area (distribution and magnitude), contact load, and contact pressure during rehabilitation should be considered to minimize the danger of damaging the graft and to support the healing process by stimulating the graft physiologically in harmless positions. The articulation and contact area at various degrees of knee flexion are of crucial importance to ACI rehabilitation in relationship to the graft location (Table 16-1).

TABLE 16-1 Summary of Patellar Articulation During Knee Flexion and Extension

The patella has a large articulating surface, presenting with the thickest layer of articular cartilage in order to optimize the distribution of forces and stresses.^13–15 The patellar cartilage presents with multiple facets in a pattern that is unique to each individual, and it does not follow the contour of the underlying subchondral bone.¹³ The articular surface of the joint is congruent in the axial plane but not in the sagittal plane, and the material properties of the patellar cartilage differ from those in the cartilage of the articulating trochlea.^5,13

The kinematics of the tibiofemoral joint is initiated, guided, and limited mainly by the cruciate ligaments, muscles, and capsular structures. Injury or loss of function to one of these structures leads to altered arthrokinematics, which may be deleterious to the menisci and cartilage.¹⁶

During normal activities, the joint contact forces (shear and compressive forces) that are produced are attenuated by several structures of the joint. Shear forces are primarily restrained by the cruciate ligaments. Compressive forces are mostly attenuated by the menisci and the articular cartilage.^5,17 Excessive shear and compressive forces can be deleterious to the menisci and the cartilage. Numerous studies have measured these forces;¹⁶ the exact level of musculoskeletal loading is influenced by a number of individual factors such as weight, gender, movement coordination, and the activity being undertaken.

To develop a safe and effective ACI rehabilitation program, shear forces have to be minimized, and the size and location of the defect have to be known because during several activities only parts of the femur/tibia are articulating.^5,18–21 For example, the posterior aspect of the medial femur condyle contacts the tibia between 90° and 120°; therefore, loading in positions between 0° and 80° of knee flexion are unlikely to be injurious to an ACI in this area.

Neuromuscular reeducation is a critical component in the restoration of functional joint stability. Neuromuscular function broadly involves the detection of afferent input via mechanoreceptors locally in the joint, the processing of a motor response to the stimulus in the central nervous system, and the initiation of an efferent reaction to maintain balance, stability, and mobility.²² Rehabilitation can assist in the restoration of proprioception, but high-level studies are scarce.^23–25

Neuromuscular control and retraining involves varying movement speed from slow movements that target the feedback system in the early stages of rehabilitation through progressions to quick movements that focus more on retraining the feed-forward system in the later stages of rehabilitation.

The exercises should be performed throughout the full available ROM and should be performed on both the involved and the uninvolved limbs because of the likelihood that proprioception deficits are also present in the contralateral limb.^13,26–29 Specific exercises for neuromuscular rehabilitation after ACI should be addressed on an individual basis in line with any weight bearing or ROM restrictions that may be in place.

Generally, proprioceptive challenges tend to be introduced through balance training and progressed in the following ways:

In addition, it is essential that more functional, dynamic tests are incorporated into the rehabilitation program. These tests involve working with the patient on the quality of his or her neuromuscular control in activities such as descending stairs, gait, rising from chairs, and in the later stages, running, hopping, and jumping.

Hydrotherapy

Exercises in water allow early active mobilization and early loading and improve neuromuscular performance, especially during the initial phase of a rehabilitation program.^15,25,30,31

The reduction in gravity under water decreases the deleterious effects of weight bearing and dissipates the impact forces on joint structures during movement,^13,32 enabling ROM exercises to be performed in a functional position with a reduced risk of high shear forces under compression. Factors such as water depth and flow will also influence the loading demands on the knee joint, so it is important to base the rehabilitation program on the general principles of hydrotherapy.

Exercises under water produce lower electromyography (EMG) activity during isometric and dynamic conditions when compared to similar exercises on dry land,³³ thereby leading to lower joint forces. Research has shown that an early and intensive application of hydrotherapy for improving coordination and strength during rehabilitation is advisable.³⁴ In addition, moving in water endows patients with a “feeling of freedom,” as they can walk without assistive devices and move around without restriction. This is an important psychological advantage.

Manual Therapy After ACI

Two conceptual approaches to manual therapy need to be mentioned within ACI rehabilitation: the clinical investigation and the application of manual techniques to reestablish physiological mobility.

The ability to define passive movement disorders in a joint, the localization of swelling, the involvement of anatomical structures, and temperature, are necessary for good clinical practice and for a comprehensive tailoring of the rehabilitation.^5,35,36

Manual therapy as an independent application of manual techniques for general knee disorders is questionable. However, the combination of manual therapy with exercises and specific manual techniques for the enhancement of ROM prove to be more effective than exercises alone.^2,37,38

Manual therapy is often cited as being used to facilitate the restoration of local function, and ACI rehabilitation protocols often mention gentle manual mobilization techniques to prevent adhesive scar tissue formation.

Electrotherapeutic Modalities

The role of electrotherapeutic modalities in postoperative ACI rehabilitation is controversial. In the first few weeks after ACI, rehabilitative exercises are often difficult to perform, not only because of edema and pain but also as a result of the joint receptor feedback disruption that is an inevitable consequence of surgical intervention.

The proposed therapeutic benefits of electrotherapy include pain reduction, increased ROM, reduced edema, enhanced voluntary muscle recruitment, and the promotion of cartilage healing. However, research remains limited and is often restricted to animal studies, and to date, the effect of electrotherapy on chondrocytes and their maturation in vivo is largely unknown.

Therapeutic Ultrasound and Laser

Low-intensity pulsed ultrasound (LIPUS)^28,34,39 and low-level laser therapy^17,25,40,41 have been proposed as providing appropriate stimuli for the acceleration of chondrogenesis. Naito et al. studied the effect of LIPUS on cartilage in a rat osteoarthritis (OA) model using serum biomarkers such as CTX-II (type II collagen degradation) and CPII (type II collagen synthesis). CPII was significantly increased in +LIPUS group compared to −LIPUS and the sham control group. In addition, the histological damage on the cartilage (Mankin score) was ameliorated by LIPUS, and type II collagen was immunohistochemically increased by LIPUS in the cartilage of an OA model. Of interest, mRNA expression of type II collagen was enhanced by LIPUS in chondrocytes. Low-intensity pulsed ultrasound affects human articular chondrocytes in vitro.³²

Korstjens et al. investigated whether utilizing LIPUS stimulated chondrocyte proliferation and matrix production in explants of human articular cartilage obtained from donors suffering from unicompartimental osteoarthritis of the knee.⁴²

Chondrocytes were exposed to LIPUS (30 mW/cm2; 20 min/day, 6 days). Stimulation of [35S]-sulphate incorporation into proteoglycans by LIPUS was 1.3-fold higher in degenerative than in collateral monolayers and 1.9-fold higher in explants. LIPUS increased the number of nests containing four to six chondrocytes by 4.8-fold in collateral and by 3.9-fold in degenerative explants. This suggests that LIPUS stimulates chondrocyte proliferation and matrix production in chondrocytes of human articular cartilage in vitro.⁴²