The transitional phase is characterized by early maturation of chondrocytes that become less vulnerable to shear forces. At this point, the growing cartilage patch is spongy, and often soft, but durable enough to tolerate increased compressive forces [16]. Physical therapy during phase II should include proprioceptive exercises, isometric and then eccentric strengthening, and closed kinetic chain presses on a progressive basis (Fig. 16.2). The stationary bicycle should be continued with gradually increased resistance. The goal of rehabilitation in phase II is to continue strengthening the periarticular muscles and increasing proprioception in preparation for more high-demand activity in phase III.

At 3 months postoperatively, the graft is becoming firm while it continues to mature. At this point, walking distance and speed can be increased as tolerated by the patient. The goals in this phase are to increase active strengthening and to continue progressive proprioception and coordination training especially in weight-bearing positions (Fig. 16.3). As strength and endurance continue to improve, the patient can gradually return to jogging and running by 6 months. At the end of phase III, patients are assessed for progress in anticipation of possible advancement to light sports-specific activity. To progress to stage IV, patients should demonstrate no pain or swelling after 30 min of weight-bearing exercise as well as full and pain free range of motion.

Remodeling and maturation of the graft can continue for up to 2 years post operatively [14, 38]. However, by 8 months, the graft is considered stable and mature enough to tolerate sports and higher impact activities. Rehabilitation can now be focused on cross-training and return to sport (Fig. 16.4). Therapists can increase training intensity, load, and volume while maintaining a focus on proper/safe technique. Adequate periods of rest are important as athletes likely remain deconditioned compared to preoperative levels. Progression can continue to occur as long as patients have no pain or swelling after specific activities. Generally, the earliest time for return to unrestricted high-impact activity is 52 weeks.

This four stage protocol and its associated timeline provide the necessary balance between graft protection and return to full strength, range of motion, and function of the joint [31]. The effectiveness of the protocol is further enhanced when it is administered by a knowledgeable physical therapist who is familiar with specific precautions and expectations associated with ACI.

For those patients who fail arthroscopic microfracture of OTL, matrix-induced autologous chondrocyte implantation (MACI) has possibilities with its potential to generate hyaline-like cartilage [3, 5, 39]. MACI is a second-generation ACI technique used for the treatment of osteochondral defects. Specifically, chondrocytes are harvested from the patient’s damaged cartilage [5, 11] during their initial ankle arthroscopy and amplified up to 20–50 times via cell culture. These chondrocytes are then placed onto a 3-dimensional collagen matrix and implanted on the articular defect with fibrin glue [5, 9, 13–15, 19, 26, 30, 35, 37]. This procedure has been performed via an arthrotomy [18, 19]. It can also be performed using an osteotomy of the medial or lateral malleolus. However, osteotomy adds morbidity and can negatively affect patient outcomes [11]. Arthroscopic techniques have also been reported for MACI insertion [14].

Regardless of the technique, postoperative rehabilitation needs to consider the gradual maturation of the repair tissue when designing a postoperative program [16]. As with ACI, the healing tissues associated with MACI must be protected but also stimulated to allow maturation and remodeling. Initial strengthening and range of motion (ROM) exercises are important and should be performed under a controlled environment with gradual progression to limit joint reaction forces and possible shear forces. Joint movement will aid in cartilage growth through the diffusion of the synovial fluid and changes in intra-articular pressure, providing a stimulus for chondrocytes thus promoting healing and maturation [22]. As the healing MACI implant progresses through the stages of healing, the cartilage can accommodate a greater amount of force, tension, and impact. It takes 12–24 months for cartilage tissue to be fully mature [34], which makes the proper rehabilitation very important to a successful long-term outcome.

The short term goals (0–12 weeks) of this protocol work on slowly progressing weight bearing while preventing the deleterious effects of immobilization and rest, including arthrofibrosis, joint adhesions, muscle atrophy, and pain. The long-term goals (>12 weeks) are focused on returning the patient to a normal gait pattern. This includes normal weight-bearing and movement restoration. The important aspects of this protocol are range of motion (ROM) exercises, weight-bearing, strengthening, and the continual progression of these. The goal is to continue to increase the patients’ level of function over 12 months and return them to their previous level of pain-free activity.