Karin A. Payne

Injury to the growth plate, a cartilage tissue found at the ends of children’s long bones that is responsible for bone elongation, can result in bony repair tissue (or bony bar). The bony bar connects bone on both sides of the growth plate, preventing normal bone elongation. This can lead to abnormal growth and if severe, complete growth arrest. While surgical resection of this bar is performed clinically, the interpositional materials used to replace it are susceptible to bony bar reformation. Here, we developed a novel interpositional material based on a biodegradable, in situ photopolymerizable, biomimetic hydrogel with tethered TGF β 3. In vitro studies with rat mesenchymal stromal cells (MSCs) encapsulated in the hydrogel and cultured for 9 weeks in vitro showed that tethered TGF β 3 led to a hyaline-like cartilage with collagen type II, while soluble TGF β 3 produced tissue characteristic of fibrocartilage, confirmed by collagen type I. Treatment of a rat growth plate injury with the TGF β 3-tethered hydrogel with or without MSCs decreased bony bar formation and showed evidence of cartilage-like tissue after four weeks, while addition of MSCs promoted limb lengthening. This hydrogel with tethered TGF β 3 offers an innovative treatment for growth plate injuries by acting as an interpositional material that resists bony bar formation and restores growth.

Introduction: Childhood fractures involving the physis potentially result in premature physeal closure that can lead to growth disturbances. Growth disturbances are challenging to treat with associated complications. Current literature focusing on physeal injuries to lower extremity long bones and risk factors for growth disturbance development is limited. The purpose of this study was to provide a review of growth disturbances among proximal tibial, distal tibial, and distal femoral physeal fractures. Methods: Data were retrospectively collected from patients undergoing fracture treatment at a level I pediatric trauma center between 2008 and 2018. The study was limited to patients 0.5 to 18.9 years with a tibial or distal femoral physeal fracture, injury radiograph, and appropriate follow-up for determination of fracture healing. The cumulative incidence of clinically significant growth disturbance (CSGD) (a growth disturbance requiring subsequent physeal bar resection, osteotomy, and/or epiphysiodesis) was estimated, and descriptive statistics were used to summarize demographics and clinical characteristics among patients with and without CSGD. Results: A total of 1,585 patients met the inclusion criteria. The incidence of CSGD was 5.0% (95% confidence interval, 3.8% to 6.6%). All cases of growth disturbance occurred within 2 years of initial injury. The risk of CSGD peaked at 10.2 years for males and 9.1 years for females. Complex fractures that required surgical treatment, distal femoral and proximal tibial fractures, age, and initial treatment at an outside hospital were significantly associated with an increased risk of a CSGD. Discussion: All CSGDs occurred within 2 years of injury, indicating that these injuries should be followed for a period of at least 2 years. Patients with distal femoral or proximal tibial physeal fractures that undergo surgical treatment are at highest risk for developing a CSGD. Level of evidence: Level III Retrospective Cohort Study.

Growth plate injuries affecting the pediatric population may cause unwanted bony repair tissue that leads to abnormal bone elongation. Clinical treatment involves bony bar resection and implantation of an interpositional material, but success is limited and the bony bar often reforms. No treatment attempts to regenerate the growth plate cartilage. Herein we develop a 3D printed growth plate mimetic composite as a potential regenerative medicine approach with the goal of preventing limb length discrepancies and inducing cartilage regeneration. A poly(ethylene glycol)-based resin was used with digital light processing to 3D print a mechanical support structure infilled with a soft cartilage-mimetic hydrogel containing chondrogenic cues. Our biomimetic composite has similar mechanical properties to native rabbit growth plate and induced chondrogenic differentiation of rabbit mesenchymal stromal cells in vitro. We evaluated its efficacy as a regenerative interpositional material applied after bony bar resection in a rabbit model of growth plate injury. Radiographic imaging was used to monitor limb length and tibial plateau angle, microcomputed tomography assessed bone morphology, and histology characterized the repair tissue that formed. Our 3D printed growth plate mimetic composite resulted in improved tibial lengthening compared to an untreated control, cartilage-mimetic hydrogel only condition, and a fat graft. However, in vivo the 3D printed growth plate mimetic composite did not show cartilage regeneration within the construct histologically. Nevertheless, this study demonstrates the feasibility of a 3D printed biomimetic composite to improve limb lengthening, a key functional outcome, supporting its further investigation as a treatment for growth plate injuries.

The growth plate is a complex cartilage structure in long bones that mediates growth in children. When injured, the formation of a “bony bar” can occur which impedes normal growth and can cause angular deformities or growth arrest. Current treatments for growth plate injuries are limited and result in poor patient outcomes, necessitating research toward novel treatments that can prevent bony bar formation and stimulate cartilage regeneration. This study investigates alginate‐chitosan polyelectrolyte complex (PEC) hydrogels as an injectable biomaterial system to prevent bony bar formation. Biomaterial properties including stiffness and degradation are quantified, and the effect that material properties have on mesenchymal stem cell (MSC) fate is quantified in vitro. Specifically, this study aims to elucidate the effectiveness of biomaterial‐based control over the differentiation behavior of MSCs toward osteogenic or chondrogenic lineages using biochemical metabolite assays and quantitative real time PCR. Further, the PEC hydrogels are employed in a rat growth plate injury model to determine their effectiveness in preventing bony bar formation in vivo. Results indicate that hydrogel composition and material properties affect the differentiation tendency of MSCs in vitro, and the PEC hydrogels show promise as an injectable biomaterial for growth plate injuries.

Background: Pediatric long-bone physeal fractures can lead to growth deformities. Previous studies have reported that physeal fractures make up 18–30% of total fractures. This study aimed to characterize physeal fractures with respect to sex, age, anatomic location, and Salter–Harris (SH) classification from a current multicenter national database. Methods: A retrospective cohort study was performed using the 2016 United States National Trauma Data Bank (NTDB). Patients ≤ 18 years of age with a fracture of the humerus, radius, ulna, femur, tibia, or fibula were included. Results: The NTDB captured 132,018 patients and 58,015 total fractures. Physeal fractures made up 5.7% (3291) of all long-bone fractures, with males accounting for 71.0% (2338). Lower extremity physeal injuries comprised 58.6% (1929) of all physeal fractures. The most common site of physeal injury was the tibia comprising 31.8% (1047), 73.9% (774) of which were distal tibia fractures. Physeal fractures were greatest at 11 years of age for females and 14 years of age for males. Most fractures were SH Type II fractures. Discussion and Conclusions: Our analysis indicates that 5.7% of pediatric long-bone fractures involved the physis, with the distal tibia being the most common. These findings suggest a lower incidence of physeal fractures than previous studies and warrant further investigation.

Purpose of Review RNA-sequencing (RNA-seq) is a novel and highly sought-after tool in the field of musculoskeletal regenerative medicine. The technology is being used to better understand pathological processes, as well as elucidate mechanisms governing development and regeneration. It has allowed in-depth characterization of stem cell populations and discovery of molecular mechanisms that regulate stem cell development, maintenance, and differentiation in a way that was not possible with previous technology. This review introduces RNA-seq technology and how it has paved the way for advances in musculoskeletal regenerative medicine. Recent Findings Recent studies in regenerative medicine have utilized RNA-seq to decipher mechanisms of pathophysiology and identify novel targets for regenerative medicine. The technology has also advanced stem cell biology through in-depth characterization of stem cells, identifying differentiation trajectories and optimizing cell culture conditions. It has also provided new knowledge that has led to improved growth factor use and scaffold design for musculoskeletal regenerative medicine. Summary This article reviews recent studies utilizing RNA-seq in the field of musculoskeletal regenerative medicine. It demonstrates how transcriptomic analysis can be used to provide insights that can aid in formulating a regenerative strategy.

Physeal injuries can result in the formation of a “bony bar” which can lead to bone growth arrest and deformities in children. Vascular endothelial growth factor (VEGF) has been shown to play a role in bony bar formation, making it a potential target to inhibit bony repair tissue after physeal injury. The goal of this study was to investigate whether the local delivery of anti‐VEGF antibody (α‐VEGF; 7.5 μg) from alginate:chitosan hydrogels to the tibial physeal injury site in rats prevents bony bar formation. We tested the effects of quick or delayed delivery of α‐VEGF using both 90:10 and 50:50 ratio alginate:chitosan hydrogels, respectively. Male and female 6‐week‐old Sprague‐Dawley rats received a tibial physeal injury and the injured site injected with alginate‐chitosan hydrogels: (1) 90:10 (Quick Release); (2) 90:10 + α‐VEGF (Quick Release + α‐VEGF); (3) 50:50 (Slow Release); (4) 50:50 + α‐VEGF (Slow Release + α‐VEGF); or (5) Untreated. At 2, 4, and 24 weeks postinjury, animals were euthanized and tibiae assessed for bony bar and vessel formation, repair tissue type, and limb lengthening. Our results indicate that Quick Release + α‐VEGF reduced bony bar and vessel formation, while also increasing cartilage repair tissue. Further, the quick release of α‐VEGF neither affected limb lengthening nor caused deleterious side‐effects in the adjacent, uninjured physis. This α‐VEGF treatment, which inhibits bony bar formation without interfering with normal bone elongation, could have positive implications for children suffering from physeal injuries.

The physis is a cartilaginous tissue in children's long bones that is responsible for bone elongation. Physeal injuries can heal with bony repair tissue known as a “bony bar,” and this can cause growth deformities. Current treatments involve surgical resection of the bony bar and insertion of inert materials in hopes of preventing bony bar re‐formation and preserving bone elongation. However, these materials frequently fail and the bony bar commonly returns. This study investigated alginate–chitosan hydrogels as interpositional materials to block bony bar formation in a rat model of physeal injury. Further, biomaterial properties such as substrate stiffness, permeability, and degradation rate were studied. Different ratio alginate:chitosan hydrogels with or without calcium cross‐linking were tested for their inhibition of bony bar formation and restoration of the injured physis. Alginate:chitosan were mixed (a) 90:10 with calcium (90:10 + Ca); (b) 50:50 with calcium (50:50 + Ca); (c) 50:50 without calcium (50:50 − Ca); and (d) 50:50 made with irradiated alginate (IA) and without calcium. We found that repair tissue was determined primarily by the in vivo degradation rate of alginate–chitosan hydrogels. 90:10 + Ca had a slow degradation rate, prevented cellular infiltration, and produced the most bony bar tissue while having softer, more permeable material properties. IA had the fastest degradation, showed high cellular infiltration, and produced the most cartilage‐like tissue while having stiffer, less permeable material properties. Our results suggest that the in vivo biomaterial degradation rate is a dynamic property that can be optimized to influence cell fate and tissue repair in physeal injuries.

Physeal injuries can lead to bony repair tissue formation, known as a bony bar. This can result in growth arrest or angular deformity, which is devastating for children who have not yet reached their full height. Current clinical treatment involves resecting the bony bar and replacing it with a fat graft to prevent further bone formation and growth disturbance, but these treatments frequently fail to do so and require additional interventions. Novel treatments that could prevent bone formation but also regenerate the injured physeal cartilage and restore normal bone elongation are warranted. To test the efficacy of these treatments, animal models that emulate human physeal injury are necessary. The rabbit model of physeal injury quickly establishes a bony bar, which can then be resected to test new treatments. Although numerous rabbit models have been reported, they vary in terms of size and location of the injury, tools used to create the injury, and methods to assess the repair tissue, making comparisons between studies difficult. The study presented here provides a detailed method to create a rabbit model of proximal tibia physeal injury using a two-stage procedure. The first procedure involves unilateral removal of 25% of the physis in a 6-week-old New Zealand white rabbit. This consistently leads to a bony bar, significant limb length discrepancy, and angular deformity within 3 weeks. The second surgical procedure involves bony bar resection and treatment. In this study, we tested the implantation of a fat graft and a photopolymerizable hydrogel as a proof of concept that injectable materials could be delivered into this type of injury. At 8 weeks post-treatment, we measured limb length, tibial angle, and performed imaging and histology of the repair tissue. By providing a detailed, easy to reproduce methodology to perform the physeal injury and test novel treatments after bony bar resection, comparisons between studies can be made and facilitate translation of promising therapies toward clinical use. Impact Statement: This study provides details to create a rabbit model of physeal injury that can facilitate comparisons between studies and test novel regenerative medicine approaches. Furthermore, this model mimics the human, clinical situation that requires a bony bar resection followed by treatment. In addition, identification of a suitable treatment can be seen in the correction of the growth deformity, allowing this model to facilitate the development of novel physeal cartilage regenerative medicine approaches.