Stem Cells in Orthopedics: A Comprehensive Guide for the General Orthopedist
The use of biologic adjuvants in the treat-ment of operative and nonoperative ortho-pedic injuries continues to expand in concert with our understanding of the acute and chronic healing process of musculoskeletal injuries. Stem cell treatments in orthopedics are among the most commonly explored options, and have found varying levels of success in promoting osseous and soft tissue healing. Basic science and translational studies have demonstrated the potential for broad application of stem cells in the treatment of a growing number of musculoskeletal injuries. Emerging clinical studies have also provided promising results, although the vast majority of studies have featured small sample sizes and limited duration of follow-up. In addition, a number of important questions remain regarding the clinical safety,treatment delivery, and overall efficacy of stem cell augmentation of injured tissue in orthopedics.
Due to their role in inhibiting the catabolic activity of matrix metalloproteinases (MMP), MSCs have been shown to have a beneficial effect in OA 18 . In a recent study, Sato et al showed that guinea pigs with age related OA treated with MSC laden hyaluronic injections had better cartilage regeneration with higher type II collagen and lower MMP content 19 . Except for a few case series which show some clinical improvement there is a paucity of trials with human subjects which study the effect of MSCs on OA20
Avascular Necrosis (AVN) of the head of femur is one of the most debilitating disorders in young patients. It is characterized by a decreased blood supply to the bone and associated increase in intraosseous pressure. The integrity of the subchondral plate is one of the most important deciding factor between head preserving (core decompression, bone grafting, femoral osteotomies) or head sacrificing (hip resurfacing/arthroplasty) procedures. Stem cells have angiogenic and osteogenic properties. Early stages of AVN are amenable to treatment with stem cell concentrate injection combined with routine retrograde procedures such as core decompression. Bone marrow aspirates administered after core decompression have been shown to be beneficial in AVN. Stem cells were isolated and used in a study by Rastogi et al where 60 hips in early stages of AVN were randomized to be treated either with core decompression and bone marrow injection or with core decompression and injection of isolated stem cells 23 . Two year follow up showed a better functional outcome and better radiographic healing in the stem cell group.
Articular cartilage is a highly specialized tissue with a poor intrinsic capacity to repair itself. The goal of any cartilage procedure is to restore its integrity so that it can withstand the wear and tear of daily activity. Focal cartilage damage: Since Pridie introduced subchondral drilling in the late 1950s, various procedures such as microfracture and abrasionplasty have been developed to recruit MSCs from adjacent bone marrow to proliferate into chondrocytes. Unfortunately, these procedures result in the formation of an inferior quality nonhyaline cartilage. Data on use of MSCs with suitable scaffolds in cartilage healing is mostly based on animal studies, with a few human case series showing improved healing and better function after autologous MSC implantation techniques 17
Rotator Cuff Repair
The number of local resident stem cells at the site of rotator cuff tear has been shown to decrease with tear size, chronicity, and degree of fatty infiltration, suggesting that those with the greatest need for a good reparative environment are those least equipped to heal.65 The need for improve- ment in this domain is related to the still relatively high re-tear rate after rotator cuff repair despite improvements in instrumentation and surgical technique.66 The native fibrocartilaginous transition zone between the humerus and the rotator cuff becomes a fibrovascular scar tissue after rupture and repair with poorer material properties than the native tissue.67 Thus, a-MSCs have been evaluated in this setting to determine if the biomechani- cal and histological properties of the repair may improve.68 In rat models, Valencia Mora and colleagues68 re- ported on the application of a-MSCs in a rat rotator cuff repair model compared to an untreated group. They found no differences between those treated rats and those without a-MSCs use in terms of biomechanical properties of the tendon-to-bone healing, but those with stem cell use had less in- flammation shown histologically (diminished pres- ence of edema and neutrophils) at 2- and 4-week time points, which the authors suggested may lead to a more elastic repair and less scar at the bone-tendon healing site. Oh and colleagues1 eval- uated the use of a-MSCs in a rabbit subscapularis tear model, and reported significantly reduced fatty infiltration at the site of chronic rotator cuff tear after repair with its application at the repair site; while the load-to-failure was higher in those rabbits with ASCs administration, it was short of reaching statistical significance. Yokoya and colleagues69 demonstrated regeneration of rotator cuff tendon- to-bone insertional site anatomy and in the belly of the cuff tendon in a rabbit model with MSCs applied at the operative site. However, Gulotta and colleagues70 did not see the same improvement in their similar study in the rat model; these authors failed to see improvement in structure, strength, or composition of the tendinous attachment site www.amjorthopedics.com July/August 2016 The American Journal of Orthopedics ® 285 B. M. Saltzman et al despite addition of MSCs. Clinical studies on augmented rotator cuff repair have also found mixed results. MSCs for this pur- pose have been cultivated from arthroscopic bone marrow aspiration of the proximal humerus71 and subacromial bursa72 with successful and reproduc- ibly high concentrations of stem cells. Hernigou and colleagues73 found a significant improvement in rate of healing (87% intact cuffs vs 44% in the con- trol group) and repair surface tendon integrity (via ultrasound and MRI) for patients at a minimum of 10 years after rotator cuff repair with MSC injection at the time of surgery. The authors found a direct correlation in these outcomes with the number of MSCs injected at the time of repair. Ellera Gomes and colleagues74 injected bm-MSCs obtained from the iliac crest into the tendinous repair site in 14 consecutive patients with full-thickness rotator cuff tears treated by transosseous sutures via a mini-open approach. MRI demonstrated integrity of the repair site in all patients at more than 1-year follow-up.
The quality of repaired tissue in primary ten- don-to-tendon and tendon-to-bone healing has long been a topic of great interest.40 The healing poten- tial of tendons is inferior to that of other bony and connective tissues,41 with tendon healing typically resulting in a biomechanically and histologically inferior structure to the native tissue.42 As such, this has been a particularly salient opportunity for stem cell use with hopes of recapitulating a more normal tendon or tendon enthesis following injury. In addition to the acute injury, there is great interest in the application of stem cells to chronic states of injury such as tendinopathy. In equine models, the effect of autologous bm- MSCs treatment on tendinopathy of the superficial digital flexor tendon has been studied. Godwin and colleagues43 evaluated 141 race horses with spontaneous superficial digital flexor tendinopathy treated in this manner, and reported a reinjury percentage in these treated horses of just 27.4%, which compared favorably to historical controls and alternative therapeutics. Machova Urdzikova and colleagues44 injected MSCs at Achilles tendinop- athy locations to augment nonoperative healing in 40 rats, and identified more native histological organization and improved vascularization in com- parison to control rat specimens. Oshita and col- leagues45 reported histologic improvement of tend- inopathy findings in 8 rats receiving a-MSCs at the location of induced Achilles tendinopathy that was significantly superior to a control cohort. Bm-MSCs were used by Yuksel and colleagues46 in compari- son with platelet-rich plasma (PRP) for treatment of Achilles tendon ruptures created surgically in rat models. They demonstrated successful effects with its use in terms of recovery for the tendon’s histopathologic, immunohistochemical, and biome- chanical properties, related to significantly greater levels of anti-inflammatory cytokines. However, these aforementioned findings have not been uniform across the literature—other authors have reported findings that MSC transplantation alone did not repair Achilles tendon injury with such high levels of success.47 Human treatment of tendinopathies with stem cells has been scarcely studied to date. Pas- cual-Garrido and colleagues48 evaluated 8 patients with refractory patellar tendinopathy treated with injection of autologous bm-MSCs and reported successful results at 2- to 5-year follow-up, with significant improvements in patient-reported outcome measures for 100% of patients. Seven of 8 (87.5%) noted that they would undergo the procedure again.
Clinical application of MSCs in the treatment of meniscal pathology is evolving as well. ASCs have been added to modify the biomechanical environ- ment of avascular zone meniscal tears at the time of suture repair in a rabbit, and have demonstrated increased healing rates in small and larger lesions, although the effect lessens with delay in repair.63 Angele and colleagues64 treated meniscal defects in a rabbit model with scaffolds with bm-MSCs compared with empty scaffolds or control cohorts and found a higher proportion of menisci with healed meniscus-like fibrocartilage when MSCs were utilized. In humans, Vangsness and colleagues30 treated knees with partial medial meniscectomy with allogeneic stem cells and reported an increase in meniscal volume and decrease in pain in those pa- tients when compared to a cohort of knees treated with hyaluronic acid. Despite promising early results, additional clinical studies are necessary to determine the external validity and broad applica- bility of stem cell use in meniscal repair.
Anterior Cruciate Ligament Reconstruction
Bm-MSCs genetically modified with bone mor- phogenetic protein 2 (BMP2) and basic fibroblast growth factor (bFGF) have shown great promise in improvement of the formation of mechanically sound tendon-bone interface in anterior cruci- ate ligament (ACL) reconstruction.80 Similar to the other surgical procedures mentioned in this review, animal studies have successfully evaluated the augmentation of osteointegration of tendon to bone in the setting of ACL reconstruction. Jang and colleagues3 investigated the use of nonauto- logous transplantation of human umbilical cord blood-derived MSCs in a rabbit ACL reconstruction model. The authors demonstrated a lack of immune rejection, and enhanced tendon-bone healing with broad fibrocartilage formation at the transition zone (similar to the native ACL) and decreased femoral and tibial tunnel widening as compared to a control cohort at 12-weeks after surgery. In a rat model, Kanaya and colleagues81 reported improved histological scores and slight improvements in biomechanical integrity of partially transected rat ACLs treated with intra-articular MSC injection. Stem cell use in the form of suture-supporting scaffolds seeded with MSCs has been evaluated in a total ACL transection rabbit model; the authors of this report demonstrated total ACL regeneration in one- third of samples treated with this augmentation option, in comparison to complete failure in all suture and scaffold alone groups.82 The use of autologous MSCs in ACL healing remains limited to preclinical research and small case series of patients. One human trial by Silva and colleagues83 evaluated the graft-to-bone site of healing in ACL reconstruction for 20 patients who received an intraoperative infiltration of their graft with adult bm-MSCs. MRI and histologic analysis Several animal studies have demonstrated improved mechanical properties and collagen composition of tendon repairs augmented with stem cells, including Achilles tendon repair in a rat model. 286 The American Journal of Orthopedics ® July/August 2016 www.amjorthopedics.com Stem Cells in Orthopedics: A Comprehensive Guide for the General Orthopedist showed no difference in comparison to control groups, but the authors’ conclusion proposed that the number of stem cells injected might have been too minimal to show a clinical effect.
Achilles Tendon Repair
The goal with stem cell use in Achilles repair is to accelerate the healing and rehabilitation. Sever- al animal studies have demonstrated improved mechanical properties and collagen composition of tendon repairs augmented with stem cells, including Achilles tendon repair in a rat model. Adams and colleagues75 compared suture alone (36 tendons) to suture plus stem cell concen- trate injection (36 tendons) and stem cell loaded suture (36 tendons) in Achilles tendon repair with rat models. The suture-alone cohort had lower ultimate failure loads at 14 days after surgery, indicating biomechanical superiority with stem cell augmentation means. Transplantation of hypoxic MSCs at the time of Achilles tendon repair may be a promising option for superior biomechanical failure loads and histologic findings as per recent rat model findings by Huang and colleagues.76 Yao and colleagues77 demonstrated increased strength of suture repair for Achilles repair in rat models at early time points when using MSC-coated suture in comparison to standard suture, and suggested that the addition of stem cells may improve early mechanical properties during the tendon repair process. A-MSC addition to PRP has provided significantly increased tensile strength to rabbit models with Achilles tendon repair as well.78 In evaluation of stem cell use for this purpose with humans, Stein and colleagues79 reviewed 28 sports-related Achilles tendon ruptures in 27 patients treated with open repair and BMAC injection. At a mean follow-up of 29.7 months, the authors reported no re-ruptures, with 92% return to sport at 5.9 months, and excellent clinical out- comes. This small cohort study found no adverse outcomes related to the BMAC addition, and thus proposed further study of the efficacy of stem cell treatment for Achilles tendon repair.