Return to Play After Knee Articular Cartilage Restoration: Surgical Options, Rehabilitation Protocols, and Performance Outcomes

Abstract

Purpose of Review

Numerous cartilage restoration techniques have proven to be effective in the treatment of articular cartilage defects. The ultimate goal of these procedures is to improve pain and function, thereby increasing the likelihood of a patient’s return to physical activity. Postoperative rehabilitation is a key component for a successful and expedient return to activities. The purpose of this article is to review the current literature regarding common surgical options, rehabilitation protocols, and performance outcomes after operative treatment of articular cartilage defects.

Recent Findings

Studies have demonstrated improved short- to long-term outcomes in a majority of techniques. However, the clinical benefits of microfracture are short-lived, which has led to the use of alternative procedures. Rehabilitation protocols are not standardized, but emphasis has been placed on bracing, weightbearing, early continuous passive range of motion, and strengthening to improve function. There is growing evidence to suggest that accelerated rehabilitation after matrix-induced autologous chondrocyte implantation may result in superior outcomes compared to delayed rehabilitation. Overall, most techniques result in satisfactory rates of return to play, though existing comparative studies typically include patients with heterogeneous pathology, complicating effective synthesis of outcomes data.

Summary

In appropriately selected patients, cartilage restoration procedures after articular cartilage injury result in favorable patient-reported clinical outcomes and high rates of return to play. While studies emphasize the critical role that rehabilitation plays with respect to outcomes after surgery, there are substantial inconsistencies in protocols across techniques.

Introduction

Articular cartilage injuries of the knee are associated with pain, mechanical symptoms, functional limitations, and, when left untreated, can lead to progressive osteoarthritis. Given the inherently limited regenerative potential of articular cartilage, chondral lesions represent a challenging injury to treat—particularly for the high-demand athletic patient who strives to return to sport. In an effort to restore normal contact forces and improve overall function of the knee joint, various surgical options have been developed which treat focal defects by inducing fibrocartilaginous healing, replacing the damaged region with an osteochondral graft, or reimplanting autologous chondrocytes that have been expanded ex vivo.

Return-to-play (RTP) rates and ability to return to pre-injury level of sport performance after surgery are commonly used to quantify overall clinical success. Additionally, expected functional outcomes and the necessary rehabilitation process can guide treatment selection and help establish anticipated results when counseling patients. Many studies have attempted to compare outcomes among cartilage restoration techniques, although the heterogeneity in the sample populations being evaluated and the overall lack of well-designed comparative studies make it difficult to draw definitive conclusions [1, 2, 3•]. Furthermore, although postoperative rehabilitation is a critical component of the treatment process for articular cartilage injuries, there is insufficient data regarding optimal protocols and duration of restricted weightbearing for the various surgical options [4•]. The purpose of this article is to review the available literature related to outcomes, RTP rates, and rehabilitation protocols for cartilage restoration techniques.

Cartilage Restoration Techniques

Bone Marrow Stimulation

Although microfracture was commonly used to treat cartilage lesions in the early 2000s, the procedure has largely fallen out of favor due to poor outcomes long-term and superior alternatives. Briefly, the aim of microfracture is to perforate the subchondral bone plate, thereby leading to release of pluripotent bone marrow-derived mesenchymal stem cells (MSCs) (Fig. 1) [5]. Migration of these cells to the lesion site promotes fibrin clot formation and the stimulation of a fibrocartilaginous repair tissue [6]. Fibrocartilage is predominantly composed of type I collagen, as opposed to type II collagen found in native hyaline cartilage [6]. Thus, the fibrocartilaginous repair tissue is unable to replicate the viscoelastic properties of hyaline cartilage [7]. The advent of techniques that directly replace hyaline cartilage or stimulate autologous chondrocyte growth have been favored in recent years.

Fig. 1.

A Articular cartilage defect of the medial femoral condyle. B Arthroscopic image displaying subchondral perforation with microfracture technique. Lesion is visualized until blood and marrow components are observed migrating into the defect site

In general, indications for microfracture include patients with small (<2 cm²), full thickness symptomatic lesions without significant bone loss [8]. However, patient-specific factors such as defect size (≥2 cm²), older age (>35 years), body mass index (BMI) (≥25 kg/m²), and lesion location (weight-bearing region) are associated with worse outcomes [9, 10•].

Although microfracture may provide short-term improvement, longer-term outcomes are generally suboptimal. Studies have shown a decline in beneficial effects after 2 years, high reoperation rates, and poor long-term functional outcomes [11–13]. A systematic review by Mithoefer et al. showed that while functional outcome measures significantly improved at 2 years, 47% to 80% patients reported decreased functional scores between 18 and 36 months [12]. This is corroborated by a recent systematic review by Orth et al. that demonstrated improvement in pain and knee functional outcomes at 2 years postoperatively with subsequent decline thereafter [14]. At the 10- and 15-year follow-up period, patients experienced further decline in functional scores, with only a portion returning to their pre-operative functional status.

Microfracture has also been directly compared to alternative cartilage restoration procedures. In a randomized controlled trial of 40 patients, Solheim et al. found that patients that underwent microfracture for symptomatic full-thickness cartilage defects reported inferior outcomes than those who underwent mosaicplasty at 15-year follow-up [15•]. A meta-analysis of trials comparing microfracture to ACI demonstrated mixed results over the short- and intermediate-term periods, with no differences between groups based on some measures of functional outcomes and superior results following ACI on other measures [16]. However, the included studies used primarily older generation ACI techniques so interpretation of this analysis with respect to the improved current generation ACI technology is limited.

Deteriorating benefits over time have brought into question the utility of microfracture, with some authors suggesting discontinuation of the technique given currently available treatment options [17, 18]. In addition to relatively high rates of failure, prior microfracture may predispose patients to poorer surgical outcomes following revision procedures. Specifically, Minas et al. found that patients undergoing revision ACI after a failed marrow stimulation procedure had a significantly higher rate of failure (26% vs 8%) compared to those without a previous procedure [19]. This may be due to the disruption of subchondral bone architecture that can lead to subchondral sclerosis, subchondral cyst formation, and poor quality of tissue infill [20].

To address the limitations of microfracture, current research has been focused on augmentation techniques to enhance the quality and durability of tissue repair. Injectable orthobiologics, such as platelet-rich plasma (PRP), hyaluronic acid (HA), and MSCs, are among the most commonly studied [21]. Scaffold matrix techniques, such as bone marrow aspirate concentrate (BMAC) and hypoimmunogenic umbilical cord blood-derived MSCs, have also been investigated for their efficacy [22–24]. While some studies suggest improved outcomes with the incorporation of adjuvant treatments, results are inconsistent and often unclear due to a scarcity of high-quality studies [25]. Higher level of evidence studies will be required to assess these effects.

Postoperative Rehabilitation: Microfracture

The current body of literature related to microfracture rehabilitation is vastly heterogenous and devoid of a standardized protocol. In a review of 18 institutional protocols, Crowley et al. showed that CPM use after surgery, restriction of knee flexion, and bracing of patellofemoral lesions are among the most implemented interventions [26]. The authors reported inconsistencies in the reporting of time to therapeutic exercise, type of exercise, weight-bearing (WB) restrictions, and time to return to sport.

For patients with condylar lesions treated with microfracture, we recommend touchdown WB for 6–8 weeks without a hinge brace. This minimizes restriction of the knee while still protecting the immature fibrocartilage bed at the defect site. In a primate study, Gill et al. demonstrated remodeling of lesion infill between 6 and 12 weeks following microfracture [27]. This suggests that defect protection during this period is necessary for tissue growth. Patients should use a continuous passive motion (CPM) machine for 6–8 h per day. Beginning range of motion (ROM) exercises early is consistent with current microfracture literature, which recommends starting ROM protocols within the first week of surgery [4•]. Isometric quadriceps/hamstring contractions are encouraged. At 8–12 weeks, patients may advance to full weightbearing as tolerated (WBAT) with discontinuation of crutch use. Concurrently, stationary cycling and closed chain exercises with full ROM may begin. At 3–6 months, patients may progress strength training exercises, begin sport-specific rehabilitation, and gradually return to athletic activity as tolerated.

For patellofemoral lesions, patients should be WBAT in a hinged knee brace locked in extension for 8 weeks. This prevents excessive contact forces in the patellofemoral joint at high knee flexion angles. Patients begin ROM exercises with the CPM machine, ranging from 0 to 40° during this time. At 8–12 weeks, patients can advance to WBAT and discontinue use of crutches and hinged knee brace. The proceeding phases of rehabilitation are similar to that of patients with condylar lesions, with a predominant focus on increasing ROM and strength.

Osteochondral Autograft Transfer

For the treatment of small- (<2 cm²) to medium-sized (2–4 cm²) osteochondral lesions, the osteochondral autograft transfer system (OATS) procedure involves harvesting an osteochondral plug from a non-weight-bearing region of the patient’s knee and transferring the healthy tissue to fill the symptomatic cartilage defect (Fig. 2) [28]. Depending on the specific size and shape of the defect, multiple plugs can be harvested and implanted in a mosaic pattern (mosaicplasty). The goal is to create a congruent articular surface between the graft and adjacent cartilage, which is crucial for satisfactory outcomes. Grafts that sit 0.5 to 1 mm proud exhibit increased contact forces during knee flexion [29•] while grafts that are recessed may be associated with excessive tamping force which is detrimental to chondrocyte viability [30].

Fig. 2.

A Osteochondral lesion of the medial femoral condyle on arthroscopic imaging. B Osteochondral bone plug fully seated within defect and flush with surrounding cartilage

The indication for OATS consists of a symptomatic, small-sized (< 2 cm²), full thickness osteochondral lesion. OATS is a single-stage procedure with the advantage of integrating a graft with intact hyaline cartilage and underlying subchondral bone. Additional advantages of an autograft include direct bony healing and a lack of possible immune response. However, concern over donor site morbidity with treatment of larger lesions that require a sizable graft or multiple grafts limits the use of OATS to smaller lesions [31]. Thus, we reserve OATS for symptomatic osteochondral lesions less than 2 cm² which can be treated with 1–2 autograft plugs.

Gudas et al. compared long-term outcomes between OATS and microfracture in a cohort of 57 patients [32]. The OATS group demonstrated significantly higher outcome scores at 3 and 10 years, and lower failure rates (14% vs 38%) than microfracture. Additionally, graft fill on magnetic resonance imaging (MRI) at 10 years was significantly higher in the OATS group. Solheim et al. showed that graft survival rates remained greater than 80% for the first 7 years after OATS and remained above 60% after 5 years [33]. However, survival dropped below 80% within 12 months after microfracture. When compared to ACI, OATS may provide comparable results in short-term follow-up, though long-term data is limited [34].

Achieving surface congruence in the patellofemoral joint remains difficult because of the asymmetric topography of the articulating surface and the thickness of patellar cartilage. However, when adequate surface congruency is achieved, patellar OATS leads to good results [35•]. Recently, Figueroa et al. reported on 26 patients that underwent OATS for high-grade patellar chondral defects, finding good-to-excellent outcome scores at a mean follow-up of 2.5 years [36]. When examining MRIs obtained 6 months postoperatively, nearly 70% of patients had a complete hypertrophic repair of their defect. Furthermore, a systematic review by Donoso et al. showed a >90% graft integration at 12 months with high surface congruence among patients with high-grade patellar lesions treated with OATS [37].

Postoperative Rehabilitation: OATS

Similar to microfracture, there is no current standard rehabilitation protocol for OATS. The transfer of an osteochondral plug provides the advantage of an immediate mature cartilage bed allowing for quicker return to full WB compared to microfracture and ACI. Werner et al. evaluated outcomes after an accelerated postoperative rehabilitation protocol [38•]. Patients were permitted up to 50% WB immediately postoperatively, progression to full WBAT at 4 weeks, and full athletic activity at 6 to 10 weeks if tolerated. Although 100% of patients returned to play in less than 3 months on average, 25% reported consistent pain that required an aspiration or injection. These results suggest that while accelerated return to play is possible in patients receiving an osteochondral plug, there remains a critical period of protection necessary for adequate and sustained symptom relief.

In a review of 16 institutional rehabilitation protocols, Crowley et al. reported the use of a CPM machine, restriction of knee flexion, and bracing among the most common implements recommended following OATS and OCA [39]. There was high variability in the type of exercise, time to return to exercise, WB restrictions, and return to sport timing across protocols.

In the first 6 weeks after surgery, we recommend patients be NWB. During the first week, patients can use a hinged knee brace locked in extension. At 2–6 weeks, the brace is opened gradually (10–20° per week), and the brace is removed once sufficient strength for a straight leg raise without an extension lag is achieved. Like microfracture, the CPM machine is used for 6–8 hours a day for 6–8 weeks. During this time, stretching, patellar mobilization, isometric strengthening exercises, and stationary biking for ROM are encouraged. At 6–8 weeks, patients can advance to full WBAT and discontinue crutch use. Once painless full ROM is achieved, closed chain exercises, gait training, and unilateral stance activities may begin. At 8–12 weeks, once full WB and full painless ROM is achieved, patients can advance to proprioceptive exercises and sport-specific rehabilitation. Patients may begin light jogging at 3 months as tolerated and RTP at 6 months is commonly recommended [4•].

Osteochondral Allograft Transplantation

Osteochondral allograft (OCA) transplantation shares similarities to OATS in that the defect site is filled with an osteochondral bone plug (Fig. 3). However, because an OCA is harvested from a cadaveric donor, this technique can be used to treat larger lesions than OATS. Following a 14-day screening of the graft for infectious pathology, the procedure is scheduled as soon as possible [40]. We prefer to use a fresh graft between 14 and 28 days after harvest due to a marked decline in chondrocyte viability observed after this time [41]. Orthotopic condylar graft matching is preferred to minimize surface incongruity. However, Wang et al. provided evidence of comparable surface matching and similar clinical outcomes when comparing non-orthotopic to orthotopic OCA for condylar lesions [42]. These findings provide alternative graft options for expeditious matching and subsequent surgical treatment.

Fig. 3.

A Intraoperative images of a medial femoral condyle defect site that has been sized, reamed to the appropriate depth, and edges of the defect prepared to receive graft. B Osteochondral allograft bone plug is pressed-fitted into recipient site and thoroughly assessed for any surface incongruence

The ideal candidate for OCA is a patient with a medium to large (2–4 cm² or >4 cm²) full-thickness chondral or osteochondral defect with underlying bone involvement. Lesions without bone involvement may be treated with OCA or autologous chondrocyte implantation (ACI), particularly larger lesions not amenable to microfracture or OATS. Theoretical disadvantages include potential disease transmission and immunological graft rejection, though these risks are minimal due to improved sterility protocols [64]. Graft availability and cost are the most important limitations to treatment [43].

Clinical outcomes following OCA have consistently demonstrated satisfactory long-term improvements in function and good durability. In a case series of 180 patients, Frank et al. found a graft survival rate of 87%, though they noted a relatively high reoperation rate of 37% at a mean 5-year follow-up [44•]. The majority of patients indicated for reoperation underwent debridement within 2 years of index OCA and reported significantly inferior scores compared to patients that did not have additional surgery. A systematic review of 19 studies by Familiari et al. showed diminishing yet relatively high mean survival rates from 5 to 20 years. Similarly, they reported high reoperation rates (30.2%) and an overall failure rate of 18.2% at final follow-up [45].

Postoperative Rehabilitation: OCA

Rehabilitation protocols following OCA vary widely. Studies have demonstrated that more experienced surgeons tend to prefer a less restrictive rehabilitation approach [42, 46]. Stark et al. recently reviewed 62 studies on OCA rehabilitation and found bracing, limiting WB until after 6 weeks, and allowing early ROM to be among the most commonly recommended protocols [47•]. However, there was no standardization of timing, metrics for evaluation of progression, or clear account for changes in rehabilitation due to concomitant procedures among the studies included.

We recommend a 6-week period of NWB with a hinged knee brace initially locked in extension. The brace should be opened in 20° weekly increments and the brace can be discontinued once a straight leg raise without an extension lag can be performed. ROM is increased gradually and coupled with patellar mobilization exercises, quadriceps sets, and straight leg raises for strengthening. A CPM is used to aid in ROM recovery. At 6–8 weeks, partial weightbearing and stationary bike exercises are permitted. At 8–12 weeks, patients may gradually return to full WB. Closed chain and unilateral exercises are beneficial during this time. Patients can WB fully and gradually progress to sport-specific rehabilitation and jogging at 3 to 6 months. Sport activity may commence after 9–12 months when full painless ROM, stability, and strength have been achieved.

Autologous Chondrocyte Implantation

ACI is a cell-based restoration technique that aims at restoring hyaline-like cartilage, as opposed to fibrocartilage obtained with microfracture. ACI is a two-staged procedure that begins with a cartilage biopsy obtained arthroscopically on primary diagnostic evaluation followed by ex vivo chondrocyte expansion. In earlier generations of ACI, cultured chondrocytes were injected underneath a periosteal or collagen membrane patch during the second staged procedure. However, due to concerns over periosteal hypertrophy and graft delamination, third-generation matrix-induced autologous chondrocyte implantation (MACI) was developed [48]. MACI relies on a three-dimensional scaffold upon which chondrocytes are directly seeded within the margins of the defect avoiding rim overlay (Fig. 4) [49]. The main disadvantages of ACI/MACI are the time and costs associated with cell growth and the required 2-stage surgical procedure.

Fig. 4.

A Everted patella with stable vertical walls and debridement to subchondral bone. B Custom cutting guide used to create a graft with smooth borders from a porcine collagen I/III membrane seeded with chondrocytes. C MACI scaffold inserted into defect site with cell-seeded side facedown. Fibrin glue is used to secure the graft in place

ACI is indicated for symptomatic, medium to large (>2 cm²) full thickness chondral defects. While primarily indicated for surface lesion with an intact subchondral bone plate, up to 6–8 mm of bone loss may be treated with concomitant bone grafting, using the “sandwich” technique [19, 50]. With the sandwich technique, the subchondral bone defect is filled with autologous bone graft. One MACI scaffold is inserted with the cell-seeded side facing up followed by a second MACI scaffold inserted with the cell-seeded side facing down, completing the sandwich. Ogura et al. reported outcomes in a small cohort of 15 patients undergoing ACI with a sandwich technique [50]. They demonstrated improved outcomes, 100% graft survival, and excellent defect fill at a mean of 7.8 years postoperatively. When compared to bone grafting, this technique demonstrated superior improvement in outcome scores and lower rates of failure [19]. Given the limited sample sizes and lack of a control group in most studies using these techniques, further research with longer follow-up is still needed.

In a meta-analysis of 9 ACI studies, Pareek et al. found a graft survival rate of 82% at a mean 11.4-year follow-up [51]. Similarly, recent literature on long-term outcomes following MACI has shown favorable outcomes. Andriolo et al. reported significant increases in outcome scores at a minimum 2-year follow-up that remained stable at 5 and 10 years [49]. Kreuz et al. evaluated defect fill on MRI and long-term outcomes following MACI [52]. At the 12-year postoperative period, they found 74.3% patients with normal or near-normal morphological fill and significantly improved outcomes at final follow-up.

Postoperative Rehabilitation: ACI

Appropriate rehabilitation following ACI is crucial for graft survival. There has been recent work addressing RTP timelines for ACI with the use of accelerated rehabilitation protocols. Della Villa et al. compared clinical outcomes for MACI using a standard postoperative protocol versus those with additional isokinetic strengthening and on-field rehabilitation [53]. They found an earlier return to competition among the specialized protocol group (10.6 vs 12.4 months). Ebert et al. sought to identify differences in outcomes in MACI patients who were randomized to either accelerated (FWB at 8 weeks) or conservative (FWB at 11 weeks) protocols [54]. They found that patients in the accelerated group reported significantly less pain across different time points, as well as superior 6-min walk distance tests and improved knee extension. These findings were corroborated in a follow-up study comparing accelerated (FWB at 6 weeks) versus current best practice (FWB at 8 weeks) guidelines [55•]. The authors posited that the same biomechanical stress that leads to growth and strength in a healthy knee may also aid in the restorative process of newly forming tissue in the postoperative period.

In general, patients are initially NWB (2 weeks) with gradual progression to partial (2–4 weeks) and FWB (6–12 weeks) as ROM and strength increase. Patients should be in a hinged knee brace locked in extension with gradual (20°) increments of progressive flexion. Discontinuation of the brace can occur once a straight leg raise is performed without extension lag. Similar to aforementioned protocols, use of a CPM machine is advised soon after surgery, with progression to 90° by week 4 and 120° by week 6. Straight leg raises and quad sets are used to regain initial strength. By weeks 2–6, progressive isometric closed chain exercises and open chain strengthening may begin. Stationary bike exercises with light resistance can begin at week 10. Weeks 12–24 consisted of full WBAT with increasing painless ROM and advancement of therapeutic exercises to improve strength and proprioception. At 6–9 months advanced strength training, light plyometric exercises, treadmill jogging, and sport-specific training can begin. From 9–12 months, patients are advised to continue strengthening with gradual integration into impact sports at about 12 months.

Key Principles of Postoperative Rehabilitation

To our knowledge, there are scant randomized studies comparing different rehabilitation protocols following cartilage restoration procedures. Thus, protocols are based on information extrapolated from basic science studies, an attempt to biomechanically optimize the defect site, and surgeon experience. Proper implementations of WB, bracing, gradual progression of ROM, and therapeutic strength exercises are required to aid in chondrogenesis, prevent arthrofibrosis, increase joint stability, and mitigate muscle atrophy. The CPM machine is consistently among the most recommended tools for postoperative rehabilitation of cartilage lesions. Protocols may differ based on the location of the lesion (condylar vs patellofemoral) and whether concomitant procedures are performed. Concomitant procedures often confound variables when assessing progression through respective phases of rehabilitation, and therefore should be accounted for. A summary of protocol guidelines for each restoration procedure is summarized in Table 1.

Table 1.

Rehabilitation protocols for cartilage procedures

Protocol	Phase I (Weeks 0–8)	Phase II (Weeks 8–12)	Phase III (Months 3–6)	Phase IV (Months 6–9)	Phase V (Months 9–18)
Microfracture (femoral condyle)	Touchdown WB (20–30% of body weight max) for 6–8 weeks; CPM for 6–8 h/day, ROM exercises	Advance to full WB; ROM: full/painless	Full WB; advancing closed chain exercises, sport-specific rehab	Gradual return to athletic activity; maintenance program for strength and endurance
Microfracture (trochlea/patella)	WBAT in hinged knee brace locked in extension; CPM for 6–8 h/day, ROM exercises	Advance to full WB ROM: full/painless	Full WB; advancing closed chain exercises, sport-specific rehab	Gradual return to athletic activity; maintenance program for strength and endurance
Osteochondral allograft (OCA) transplantation	Non-WB, hinged knee brace locked in extension (week 1); CPM for 6–8 h/day, ROM exercises	Partial WB (25% of body weight); ROM: full/painless	Gradual return to full WB; advancing closed chain exercises	Full WB with normal gait pattern; sport-specific rehab; maintenance program for strength and endurance	Return to athletic activity at 9–12 months post-op
Osteochondral autograft transfer system (OATS)	Non-WB, hinged knee brace locked in extension (week 1); CPM for 6–8 h/day, ROM exercises	Advance to full weightbearing; ROM: full/painless	Full WB; advancing closed chain exercises, sport-specific rehab; gradual return to athletic activity; maintenance program for strength and endurance	Higher impact activity at 4–6 months
Autologous chondrocyte implantation (ACI) (femoral condyle)	Weeks 0–2 non-WB, weeks 2–4 partial WB (30–40 lbs), weeks 4–6 continue with partial WB; CPM for 6 h/day for 8 weeks	Full WB with normal gait pattern; ROM: full/painless	Full WB with normal gait pattern; advancing strength training	Full WB with normal gait pattern; maintenance program for strength and endurance; closed chain exercises	Sport-specific rehab at 9 months Return to impact athletics at 16 months (if pain-free)
ACI (trochlea/patella)	WB: weeks 0–2 non-WB, weeks 2–4 partial WB (30–40 lbs), weeks 4–8 continue with partial WB; CPM for 8 h/day for 8 weeks	Full WB with normal gait pattern; ROM: full/painless	Full WB with normal gait pattern; advancing closed chain exercises, sport-specific rehab	Full WB with normal gait pattern; maintenance program for strength and endurance	Sport-specific rehab at 9 months Return to impact athletics at 16 months (if pain-free)

CPM continuous passive motion, Rehab rehabilitation, ROM range of motion, WB weightbearing, WBAT weightbearing as tolerated

Return to Play

Microfracture and OATS

The ability to return to pre-injury sport activity is coveted among recreational and competitive athletes. The expected time to return to sport after microfracture is approximately 4–6 months, which may seem like a desirable characteristic compared to procedures such as OCA and ACI that have longer anticipated return times [3•, 4•]. However, satisfactory outcomes may only be short-lived [11–14]. Broad comparisons of RTP among techniques, while generally informative, fail to account for the extent of cartilage pathology that influenced initial treatment choice.

Campbell et al. found in their systematic review of 970 patients that those who underwent microfracture had the lowest return to sport rate (75%) compared to OATS (89%), OCA (88%), and ACI (84%) [1]. This study reported that microfractures for lesions less than 2 cm² were more likely to return to high impact sports. However, the mean defect size of included patients ranged from 1.55 to 7.25 cm², undoubtedly affecting treatment choices and RTP outcomes. Moreover, the wide variety of sports included in the study potentially influenced RTP due to differing levels of physical demand required of each sport. While the results of this retrospective analysis provide strong evidence that OATS, OCA, and ACI yield similarly high RTP rates, the inability to control for underlying indication bias prevents any comparison between different cartilage restoration procedures for patients who could reasonably be indicated for more than one option.

Recent systematic reviews by Robinson et al. and Hurley et al. similarly found that patients undergoing microfracture had among the lowest RTP rates (62% and 77%, respectively) compared to OCA, OATS, and ACI/MACI [3•, 4•]. The authors noted heterogeneity in the types of sports played and differing skill levels (recreational vs professional) of athletes included. While they reported rates of return to pre-injury levels, the reasons for not returning to sport could not be obtained. This creates a challenge in distinguishing barriers to RTP that are not exclusively symptom-associated. RTP was highest following OATS in both analyses, although this is likely explained by the considerable overlap in studies included in the two systematic reviews. Robinson et al. suggested that patients with smaller defects, an indication for OATS, may have less severe injury which in turn may improve the likelihood of RTP [3•]. Hence, it is important to consider that comparisons of RTP rates are only clinically applicable when comparing procedures that are used to treat similar injuries, such as microfracture versus OATS.

Han et al. conducted a recent meta-analysis of seven randomized, controlled trials directly comparing outcomes between microfracture and OATS [56]. They found that patients undergoing OATS were more likely to return to play, return to pre-injury performance levels, and return at a faster rate. Gudas et al. followed 57 competitive athletes and found that 93% of athletes undergoing OATS and 52% of those undergoing microfracture were able to return to their pre-injury levels [57]. At 10-year follow-up of the same cohort, there was a drop in sport activity in both groups, but patients in the microfracture group demonstrated a significantly lower level of sport activity compared to the OATS group [32]. Collectively, these findings suggest that for the athlete seeking to optimize their probability of regaining prolonged athletic performance after surgery, OATS is likely superior to microfracture.

OCA and MACI

Indications for OCA and ACI differ mainly based on whether there is involvement of the underlying bone. Nonetheless, there is overlap in patient selection due to variations in injury presentation and surgeon preference. Both techniques may be used for smaller defects, though they are typically reserved for defects >2 cm².

The aforementioned systematic review by Hurley et al. reported RTP following OCA and ACI to be 77% and 80%, respectively. Robinson et al. similarly reported RTP rates of 84% and 81% for OCA and ACI [3•]. Despite mostly improved outcomes and similar trends in RTP after OCA, Crawford et al. discussed three studies reporting worsening sport outcome values 2 years postoperatively [58]. Additionally, reoperation rates for OCA were high, ranging from 34 to 53%. The authors acknowledged having limited patient-specific information would preclude risk factor analysis for reoperations. Patients greater than 25 years of age, duration of symptoms >12 months, large graft size, anatomic location, or female sex may negatively affect RTP [2, 59•]. Thus, deliberate counseling of the importance of postoperative rehabilitation and setting attainable goals for patients at higher risk is essential.

Campbell et al. systematically reviewed first- and second-generation ACI and found comparable RTP rates compared to OCA [1]. Both OCA and ACI had significantly greater rates of RTP compared to microfracture. The level at which patients can participate in sports upon return is also important, particularly for the competitive athlete. Hurley et al. reported more than half (60% and 57%) of patients that return will do so at the same or higher level for both OCA and ACI, respectively [4•]. Time to return to full sporting activity also relies on many additional factors including physical aptitude, psychological readiness, and rehabilitation protocol restrictions. Campbell et al. reported a mean 9.6 months to RTP for OCA and 16 months for ACI [1]. This underscores the importance of more clinical studies investigating accelerated RTP after ACI, particularly at the collegiate or elite level, given that extended rehabilitation may deter athletes from treatment.

There are few studies that examine elite level athletes [60, 61]. Balazs et al. reported on 4 professional and 7 collegiate basketball players and found an 80% return to their previous level of competition [62]. An earlier prospective cohort study by Kon et al. reported on 41 professional or semi-professional male soccer players treated with second-generation ACI or microfracture [63]. In this study, 86% returned to competition for the ACI group compared to 80% of the microfracture group. While the ACI group took significantly longer to return than the microfracture group (12.5 vs 8 months), they had significantly better outcome scores at 2-year follow-up. We advocate for the use of both OCA and ACI in athletes given their favorable results and durability, which is important for patients expected to continue to exert repetitive forces on the knee joint.

Conclusion

Multiple cartilage restoration techniques are currently available for the treatment of symptomatic chondral lesions in the knee. Although the majority of techniques have demonstrated satisfactory functional outcomes after surgery, microfracture use has diminished given reports of waning long-term durability. Although postoperative rehabilitation is crucial, there is scant clinical evidence demonstrating superiority of any one protocol. While generally good-to-excellent, RTP rates are heterogeneous and often compared among techniques with different indications and therefore inherently different patient populations. Under the guidance of established surgical indications, clinical trials comparing various rehabilitation protocols are warranted to demonstrate the most efficacious method to improve patients’ ability to return to pre-injury level of sport activity. Lastly, standardized metrics to evaluate preparedness to advance through rehabilitation phases are needed to safely guide progression.

Declarations

Conflict of Interest

Jairo Triana, Zachary I. Li, Naina Rao, and Matthew T. Kingery declare that they have no conflict of interest relevant to this article. Eric J. Strauss is a paid consultant to Subchondral Solutions and Vericel, and receives support from Cartiheal and Springer Publishing Company.

Human and Animal Rights and Informed Consent

This article does not contain any studies with human or animal subjects performed by any of the authors.