|Biomechanical Effects of Meniscal Allograft Fixation: A Physiological Model
Gee, AO; Chen, T; Hutchinson, I; Stoner, K; Wanivenhaus, F; Warren, R; Maher, SA
Hospital for Special Surgery, New York, NY, USA
Introduction: Allograft meniscal transplantation is a treatment option for patients who have pain and dysfunction due to severe and irreparable tears of the meniscus . There are many variables that can affect the long-term performance of allografts (including graft geometry, level of activity of the patients, limb alignment [2,3]); but the variable that is most directly under the control of the surgeon is the method of graft fixation. The most common method of fixation involves trans-osseous suture fixation via bone plugs; where the bone plugs are machined at the anterior and posterior horns of the graft and implanted into appropriately sized tibial bone tunnels. Other techniques include suture fixation at the meniscal horns; where the sutures are drawn through tibial bone tunnels and tied over a bone-bridge. But the mechanical consequences of these fixation techniques, specifically, how they affect the ability of the graft to distribute forces across the knee joint under physiological activities, immediately after implantation and as a function of time thereafter, are unclear. The objective of this study was to quantify the contact mechanics associated with different graft fixation techniques. Our hypotheses were that: (A) allograft implantation would improve contact mechanics relative to that of the meniscectomized knee, regardless of method of fixation (B) bone plug fixation with sutures would restore contact mechanics to that of the intact, un-operated knee, while suture-only fixation would not and (C) bone ingrowth would further improve mechanics.
Methods: The Model: A load controlled, dynamic, multidirectional simulator (Stanmore, Middlesex, UK) was used to mimic the forces experienced during gait across cadaveric knees . The knees were augmented with a thin electronic sensor (Tekscan, MA), placed under both menisci and attached to the tibial plateau to dynamically measure contact stresses and contact area throughout testing (n=4 knees). Conditions Tested: The knees were tested under the following conditions: (i) Intact, (ii) Meniscectomy, (iii) Bone Plug & Suture: allograft fixation with bone plugs secured into bone tunnels with suture, (iii) Bone Tunnel & Suture: allograft fixation with suture-only through bone tunnels, (iv) Bone Plug & Cement: allograft fixation with bone plugs secured into bone tunnels with bone cement to simulate the mechanical consequences of bone ingrowth . The order of testing each condition in each knee was specified based on the technical complexities, as follows: After the intact knee was tested, the medial meniscus was incised at the menisco-capsular junction. A 1 cm x 1 cm bone plug was created at the anterior horn attachment using an osteotome (Fig. 1A). Posteriorly, a guide pin was placed through the attachment site and an 11 mm round bone plug was created using a cannulated coring reamer (Fig. 1B). The meniscus was then repaired back to the capsule using three 2-0 Ethibond sutures placed anterior, middle and posterior in vertical mattress fashion. The anterior and posterior bone plugs were reduced into their respective bone tunnels and secured using #2 Ethibond sutures (Bone Plug & Suture group). 2 sets of sutures were placed into each horn—one through the bone plug and one through the meniscus only (Bone Tunnel & Suture). Once these conditions were tested, bone cement (PMMA in liquid state) was injected into both tunnels and the bone plugs were held in place until the cement fully hardened. Outcome Measures: Contact stress data was recorded for 20 gait cycles at a frequency of 200 Hz. The data from the sensor was extracted and using customized programs in Matlab, the contact area, and maximum force across the plateau at 14% and 45% of the gait cycle (when the axial force was at its highest) was computed. A one-way ANOVA with Tukey post-test was performed to determine significance for contact area and max force measurements at 14% and 45% of gait.
Results: Results were significantly affected by the phase of gait at which the data was analyzed (Figure 2). At 14% of the gait cycle contact area significantly decreased and total force significantly increased after meniscectomy. Regardless of fixation method, all allograft implanted conditions exhibited significantly higher contact area and significantly lower contact stress relative to the meniscectomized condition. But, the only group that restored contact area to that of the intact condition was the Bone Plug & Cement group. At 45% of the gait cycle contact area was again significantly reduced with meniscectomy, but none of the fixation methods showed a statistically significant increase in contact area relative to meniscectomy. There was no significant effect of any condition at 45% of the gait cycle.
Discussion: By way of a dynamic, multidirectional, physiological cadaveric model we successfully quantified the contact mechanics associated with different meniscal graft fixation techniques. We accepted the hypothesis that allograft implantation improved contact mechanics relative to that of the meniscectomized knee, regardless of method of fixation. However, neither bone plug fixation nor suture-only fixation restored contact mechanics to that of the intact, un-operated condition: this was despite the fact that an ‘ideal’ graft was used - i.e. the native meniscus was explanted and then re-implanted. Interestingly, when bony ingrowth was simulated, contact mechanics in early gait was similar to that of the intact condition, suggesting that improvements with bony ingrowth are possible.
Significance: Through use of a dynamic, multidirectional load controlled simulator, we found that knee joint contact mechanics in the early stance phase of gait were significantly affected by the method of fixation of meniscal allografts. Ability of the graft to carry load was improved with simulated bony ingrowth, suggesting that enhancing bony fixation may lead to improved joint function.
Acknowledgements: The authors would like to thank the NIH for providing support for this study (T32-AR007281-27). We also acknowledge the support of the Clark and Kirby Foundations and the Russell Warren Chair in Tissue Engineering.
References:  Dienst et al. Am J Sports Med 2007.  Rodeo et al. Am J Sports Med 2001.  Alhalki et al. Am J Sports Med 1999.
 Bedi et al. J Bone and Joint Surg 2010.  Saha et al. J Biomed Mater Res 1984. Figure 1: A- 1 cm x 1 cm bone plug was created at the anterior horn attachment using an osteotome. B- Posteriorly, a guide pin was placed through the attachment site and an 11 mm round bone plug was created using a cannulated coring reamer.
Figure 2: Measurements of contact area and max force at 14% and 45% of gait representing the two peak axial loads during gait.* indicates significant difference from the menisccomized condition.
ORS 2013 Annual Meeting
Poster No: 0570