In this study we have developed a cadaveric biomechanical test model for calcaneoquartal arthrodesis sensitive enough to detect differences in glue strength and have shown that a new, phosphoserine modified, bone glue adds strength to the arthrodesis, ex vivo. This is the first study to evaluate a glue for fusing together subchondral bone surfaces. Our hypothesis that by adding the glue to an arthrodesis of the calcaneoquartal joint could add stability was confirmed.
Arthrodesis involves meticulous removal of articular cartilage, placement of a cancellous bone graft for its osteoconductive, osteoinductive, and osteogenic properties, and a rigid internal or external fixation to achieve osseous fusion [17, 18]. Rigid fixation stability is the primary determinant of arthrodesis success, by promoting intramembranous ossification, while poor fixation leads to formation of fibrocartilage and non-union [19–24]. In the human metatarsophalangeal joints, movement of the surfaces by 2 mm after arthrodesis is sufficient to interrupt healing, leading to non-union [25, 26]. In the present study, joints treated with glue endured a displacement of 2–3 mm before the glue failed. In a clinical setting, the glue may be sufficient to reduce or prevent micro-movements between joint surfaces held together by orthopedic implants. A technology that reduces the risk of implant failures and non-union, especially in canine tarsal and carpal arthrodesis, where not all parts of the joint are fixated by the implant(s) could potentially be of great clinical value.
Another potential technique, not yet investigated, could be to fixate joints in correct alignment with the glue (solidifies in 60–90 seconds) before placing the implants during arthrodesis, thereby avoiding post malalignments, or misplacement of implants [6].
Historically the use of an external coaptation, usually a cast, to add rigidity and reduce the load of the internal implants has been advocated [8]. The effectiveness of coaptation remains however unclear and recent studies suggest the contrary for tarsal and carpal arthrodesis [2, 27]. A glue that ads stability to the fixation could even further diminished the need of an external coaptation in cases where it still is deemed necessary and the complications associated with that [28].
The glue evaluated in this study is a type of biomaterial: a ceramic glue that resorbs and is replaced by bone. The number of case reports evaluating biomaterials for arthrodesis are few and thus far there is no ceramic or polymeric biomaterial that can stabilize, fixate, or bond with subchondral bone [29–35]. No resorbable material has achieved regulatory approval for use in gluing or fixating subchondral bone and there are no other case reports of an adhesive that can augment, or improve, arthrodesis procedures.
The developed cadaver model was sensitive enough to distinguish between glues with different strengths, and to identify failure of glue in the presence of some soft tissues, similar to the actual clinical conditions of arthrodesis. However, the presence of intact soft tissues contributed to stability and interfered with measurements in intact joints. We conclude, therefore, that biomechanical models of tarsal joint arthrodesis, which use an adhesive to bond or fixate joint surfaces, require removal of at least proximal soft tissues to identify failure of the glue and measure bond strength. The glue appeared to participate in load sharing with nearby soft tissues, similar to what would occur in fused joints in vivo.
After curing, the glue in this study resembles a ceramic material. Flaws or defects, such as pores, limit the structural strength of ceramics [36]. The likelihood that defects arise during bonding or setting should increase with increases in the size of the bonding surface. Therefore, it was theoretically possible that the failure strength of the reconstructed joints might be correlated with the size of the joint. However, the bond strengths per square centimeter were similar for both small and large dogs.
The glue appeared strong enough to warrant further testing in joint arthrodesis, pending testing in a chronic (cyclical) loading model to account for premature failure due to fatigue. While it was only possible to evaluate the short term, immediate fixation strength (24 hours) in the present model, in a recent study we have demonstrated that the adhesive bonds to live cancellous and cortical bone (in vivo) [14] just as strongly as to cadaver bone [13] in rodents and in human bone [37]. Therefore, the results of this study are expected to closely predict how the glue will behave in live canines, within 24 hours of fixation.
In laboratory testing, on metals and cortical bone, the average adhesive strength of the glue is typically 400 N/cm2 [15]. This suggests that the joint fusion strength could be increased 2-4-fold (to reach the true strength of the glue) with additional improvements in the handling and fixation technique, or by optimizing the glue to bond with subchondral bone, rather than cortical bone, for future studies.
There were several limitations to the present model and data. First, the present cadaver model represented a compromise, between the need to remove all soft tissues and musculature to achieve a uniaxial loading regimen at a single bone/adhesive interface (e.g. pure shear). There was also a need to reproduce the clinical situation, where the injuries often are more complex, involving several joints and where the surrounding soft tissues and muscle disperse load, thereby preventing an accurate measurement of the biomechanical strength arising only from the bonded joint surfaces. Ex-vivo testing also did not consider changes in viscoelasticity between live tissue and cadaver tissue due to decomposition (cadaver), temperature, healing, inflammation, or the presence of fluids that can affect the material properties in live tissue.
Another limitation in the present study was the type of loading forces. A purely shear force was used because this is the most rigorous test for a glue (i.e. most glues are weakest in shear), in addition to being a force that arises during normal locomotion, besides compression, bending and torsion. While tarsal loading has been evaluated in general by using: 1-, 2- and 3-point bending [25, 38–40]; pure torsion [23]; compression [41]; shear [26] and multi-modal loading using intact (cadaver) ankles [42] there is no clear consensus on which type(s) of loading are sufficient to accurately model forces that arise during locomotion. No other studies have evaluated adhesives for joint fixation, and the magnitude of force(s) that arise during joint movement are poorly characterized in canines. The most comprehensive study thus far in humans, Wang et al [24] found that the expected peak load in human calcaneocuboidal joints during movement was equivalent to 9% of bodyweight (0.3–0.5 MPa or 40-100N). Directly translating the results from Wang et al., from human to canine, is impossible due to differences in movement patterns, anatomy, and functional loading angles, between bipeds and quadrupeds. However, a reasonable assumption of loading in canine calcaneoquartal joint is that it would not be greater than in humans. Based upon this assumption the expected peak loading force in a toy breed (weighing 1–3 kg) might be 1–3 N, while a medium sized dog (5–20 kg) might exert up to 20 N on the calcaneoquartal joint during movement. This range of loading forces would be much lower than the measured strength of the glue. Both the joint and surrounding soft tissues disperse load, which may reduce the forces acting upon the joint surfaces, while also complicating analysis of precisely how strong a glue must be to resist displacement/ deformation, and to ensure joint fusion.
Finally, since we evaluated a joint where the surfaces were not visible during testing, failure of the glued joint was confirmed visually when cracks appeared in the bond line. It is possible that smaller, or partial, failures could have occurred in the glue (e.g. fatigue), when it was subjected to repeated loading. Additional studies are needed to evaluate the fatigue properties of the glue. While not obvious from the data or discussion thus far, the largest challenge in this study was developing a manual fixation method to hold the uneven joint surfaces steady, and avoid disrupting the adhesive while it cured, during the 60–90 second curing/working time. The joint surfaces were not flat, nor perfectly aligned, which is a crucial impediment for any glue, which make the results of this study even more relevant.