TR Matrix™ BioScaffold: A New Substitute for Autologous Bone Graft in Tibial Tuberosity Advancement

Based on the clinical results of this small pilot study, two larger independent studies are in process at veterinary teaching hospitals.

C. M. Pullen DVM, MS, Jeff W. Kellerman DVM, MBA, and C. W. Woods DVM, MS, MBA

Correspondence: Jeff Kellerman, TR BioSurgical, LLC 863 E. Gemini Place, Chandler, AZ 85249 jkellerman@trbio.com

Acknowledgements: Charles M. Pullen DVM, MS, Animal Medical & Surgical Center, 17477 North 82nd Street Scottsdale, Arizona 85255; Dennis Keith DVM, DACVR, Medical Director, VCA Animal Referral and Emergency Center of AZ, 1648 North Country Club Drive Mesa, AZ 85201

Abstract
The authors hypothesized that a new osteopromotive tissue repair bioscaffold, TR Matrix™, could substitute the need for autologous cancellous bone graft (ACBG) in procedures involving an osteotomy. TR Matrix™ consists of a unique open polar collagen that mimics embryonic extracellular matrix and induces fetal-like repair mechanisms; previously shown to have osteopromotive properties in critical size defect models. A total of 8 neutered dogs undergoing Tibial Tuberosity Advancement (TTA) for cranial cruciate ligament rupture were enrolled in this study. The osteotomy sites of four patients were treated solely with TR Matrix™ while the other four patients were treated with ACBG harvested from the distal femur. TR Matrix™ was mixed using 1.5cc of sterile saline per 1g, and then packed into the osteotomy site. Patients were clinically assessed by the owners and veterinarians at Day 0 and Week 6 using a standardized orthopedic questionnaire. Radiographs were evaluated at Day 0 and Week 6 using a standard radiographic scoring system. The Week 6 results demonstrated that TR Matrix™, compared to ACBG, was equal in clinical outcomes. Bone healing was equal between groups, but the TR Matrix™ treated group had significant bone healing (n=4; p=0.019) over time whereas the ACBG group had no significant improvement (n=4; p=0.114). This study suggests that TR Matrix™ may be a suitable alternative to autologous cancellous bone grafting.

Introduction
Although autogenous cancellous bone grafting remains the “gold standard” for bone repair, there are limitations to this procedure including increased anesthetic time, insufficient quantity of graft, limited access to donor sites, loss of osteogenic cells, donor site pain or hemorrhage, and predisposition of the donor bone to failure1 . This has created a substantial need for an off-the-shelf substitute that is at least equivalent to cancellous bone grafting procedures. Although a variety of biomaterials have been developed to mimic the mineral or micro-physical properties of bone, they have one or more limitations including suboptimal physiological osteopromotive properties, long resorption times, potential immunogenicity, and/or a radio-opaque property which hinders accurate assessment of natural bone healing. In addition, use of osteoinductive morphogenic proteins for fracture repair in veterinary medicine has profound limitations including substantial costs, rapid diffusion in physiological environments, and questionable results2.

A new non-immungenic bioscaffold, TR Matrix™, has been recently introduced for veterinary orthopedics. TR Matrix™ mimics the early stage embryonic extracellular matrix3,4 and induces fetal like repair mechanisms including angiogenesis5, a critical factor in bone formation6. Its properties relate to a unique tertiary structure; being an open polar configuration which exposes natural amino acid sequences which initiate physiological tissue repair. Interestingly, TR Matrix™ contains no drugs or biological factors and is regulated as a medical device. Depending on the tissue type, cells that bind to the TR Matrix™ bioscaffold will integrate and increase expression of bone morphogenic protein 2 (BMP-2), transforming growth factor beta 1 and 3 (TGF-ß1 and ß3), vascular endothelial growth factor (VEGF), connective tissue growth factor CTGF), aggrecan and other repair signals resulting in cellular in–growth, extracellular matrix turnover and vasculogenesis7,8,9,10 .

The purpose of this study was to determine if TR Matrix™ could substitute the need for cancellous bone grafting. TR Matrix™. was compared to ACBG by measuring clinical and radiographic outcomes in dogs undergoing osteotomies using the tibial tuberosity advancement (TTA) technique for cranial cruciate ligament repair.2

Materials and Methods
Experimental design: This was a 42 day (+/-5 d) pilot clinical efficacy and safety study in 8 dogs undergoing TTA for cranial cruciate ligament rupture. The osteotomy sites of 4 dogs served as positive controls and received autologous cancellous bone graft from the distal femur. The osteotomy sites of the other 4 dogs served as the treatment group and received TR Matrix™. Owners were informed of the nature of the study and signed an informed consent prior to enrollment.

Case selection: Eight patients that satisfied the inclusion and exclusion criteria were enrolled in this study. The surgical procedures were conducted by a single investigator. Each patient was diagnosed with complete cranial cruciate ligament rupture by standard palpation techniques, and then confirmed during surgical repair. Patients with known bilateral cranial cruciate ligament rupture or moderate to severe osteoarthritis in the same or contralateral stifle or coxofemoral joints were excluded from the study. Patients could receive standard medical therapy including oral antibiotics, anti-inflammatories, and other necessary treatments consistent with the procedure. Patients could not receive allografts, other biomaterials, or stem cells at the fracture site.

Treatments: TR Matrix™ consisted of 1 gram of lyophilized, sterile collagen copolymer having <12EU endotoxin/mL. Test material was supplied sterile in glass vials with a rubber stopper. Under general anesthesia using an aseptic surgical approach a standard TTA technique was performed11 . TR Matrix™ was mixed using 1.5cc of sterile saline per 1g, and then packed into the osteotomy site. In order to reduce variability among patients, the titanium cages did not vary more than +/-2 mm in length and +/-2 mm in width.

Outcome measures: Owners and surgeon evaluated each patient with questionnaires on Day 0 and 42 (+/-5d). Digital radiographs were independently evaluated by a board-certified veterinary radiologist to assess bone healing of each patient’s osteotomy site at Day 42 (+/-5d).

Results
Clinical Outcomes: As expected, dogs in both groups improved clinically due to the surgical procedure. As expected, there were no statistical differences in clinical outcomes between treatment groups, as determined by the owner and veterinarian.

Radiographic Outcomes: Table 1 describes the individual radiographic findings of the two groups by patient (P) number. Bone healing (0=no healing to 4=complete healing) was evaluated by a board certified veterinary radiologist, with group means shown in Table 2. As hypothesized, bone healing occurred in both groups and statistical analysis revealed equivalence between groups. However, the improvement in bone healing was statistically significant in the TR Matrix™ treated group (p = 0.019) but not the cancellous bone graft group (p = 0.114) by the end of the study period.

Discussion
The results support that TR Matrix™, a new osteopromotive bioscaffold, can be effectively used as a substitute for ACBG and should be a consideration in surgical procedures requiring an osteotomy. A heightened interest and use in biomaterials as alternative for cancellous bone grafting during surgery has led to the development of TR Matrix™, a unique bioscaffold with osteopromotive properties. The novel embryonic like structure appears to be responsible for its profound tissue repair properties including the natural up-regulation of local tissue repair factors from infiltrating cells. The purpose of this study was to determine if TR Matrix™ could be used as a bone graft substitute, thereby satisfying a largely unmet surgical demand in veterinary medicine.

Clinically, the control and treated patients were equivalent in those attributes associated with orthopedic disabilities, a promising finding in these patients. However, one limitation to assessing clinical outcomes was that the rate of clinical improvement was only measured on Days 0 and 42. More frequent post-operative assessments would be valuable to ascertain if TR Matrix™. had any clinical advantage to post-operative recovery, which may be expected since the degree of healing should correlate with clinical improvement.

Radiographically, the TR Matrix™ group had a significant improvement in bone healing at Day 42 when compared to baseline; whereas the cancellous bone graft group had not significant improvement over time compared to baseline. These findings are consistent with the higher Day 42 scores, substantiating TR Matrix’s™ osteopromotive properties and ability to repair bone defects. As hypothesized, the between group comparisons at Day 42 were not statistically different, although the TR Matrix™. treated groups scores were higher on average and showed a trend (p<0.1) towards an improved degree of healing. This is consistent with the hypothesis that TR Matrix™ can substitute ACBG for TTA.

One limitation to the radiographic analysis was lack of a standardized radiographic gradient, so there may be limitations to the sensitivity and/or accuracy of this assessment among films. Nonetheless, the radiographic findings are impressive, since TR Matrix™. is shown to be at least equivalent to autologous cancellous bone grafting.

In summary, certain deficits of autologous grafting can be overcome with the use of this cost-effective bioscaffold. Further studies are warranted to examine clinical outcomes at more frequent intervals to determine if there are differences among groups, since it is likely that differences would be discovered earlier in the course of recovery for this particular procedure.

Images from several patients are shown below.
TR Matrix™ treated patients, (P1) and (P3), evidenced established periosteal reaction and complete bone healing at Day 42 respectively. Autologous cancellous bone graft patients (P5) and (P7), revealed no healing and bridging callus at Day 42 respectively.

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