The ankle cartilage has an important function in walking movements, mainly in sports; for active young people, between 20 and 30 years old, the incidence of osteochondral lesions is more frequent. They are also more frequent in men, affecting around 21,000 patients per year in USA with 6.5% of ankle injuries generating osteochondral lesions. The lesion is a result of ankle sprain and is most frequently found in the medial location, in 53% of cases. The main objective of this work was to develop an experimental and finite element models to study the effect of the ankle osteochondral lesion on the cartilage behavior. The right ankle joint was reconstructed from an axial CT scan presenting an osteochondral lesion in the medial position with 8mm diameter in size. An experimental model was developed, to analyze the strains and influence of lesion size and location similar to the patient. The experimental model includes two cartilages constructed by Polyjet™ 3D printing from rubber material (young modulus similar to cartilage) and bone structures from a rigid polymer. The cartilage was instrumented with two rosettes in the medial and lateral regions, near the osteochondral region. The fluid considered was water at room temperature and the experimental test was run at 1mm/s. The Finite element model (FE) includes all the components considered in the experimental apparatus and was assigned the material properties of bone as isotropic and linear elastic materials; and the cartilage the same properties of rubber material. The fluid was simulated as hyper-elastic one with a Mooney-Rivlin behavior, with constants c1=0.07506 and c2=0.00834MPa. The load applied was 680N in three positions, 15º extension, neutral and 10º flexion.Introduction
Materials and Methods
The Temporomandibular joint (TMJ) is a complex and important joint for daily activities, and the alloplastic implant is recommended as the best solution, after repeated surgeries, failed autogenous grafts, highly inflammatory metabolic arthritis, fibrous or bony ankyloses. Some complications in total TMJ replacement are associated with implant design, screw fixation failure, implant displacement, fibrous tissue formation, (Speculand, et al. 2000). Some numeric studies evaluate the number of screws needed to guarantee the good fixation and suggest a minimum of three (Ramos et al. 2015), but is a controversy conclusion. The Biomet Microfixation TMJ stock prosthesis, Jacksonville, FL, USA is one of the three or four in the market. Clinical studies published by this device between 2005 and 2015 indicate a success rate of around 84 to 91% with improvements in mouth opening, a decrease in pain score and improved quality of life. The present study analyses experimentally the load transfer of this device. The intact, clean cadaveric ramus was instrumented with four rosettes model (KFG-1-120-D17-11 L3M2S, by Kywoa Electronic Instruments Co™, Japan), one in lateral region, two in lateral region and one in lingual face. The condyle was loaded with the temporal reaction; the load was applied constant velocity of 1mm/min in three continuum phases and with three stops at 100N, 200N and 300N. Next, the Biomet microfixation implant was fixed to the same cadaveric mandibular ramus after resection. The implant was 50mm in length. It was fixed with five 6AL/4V Titanium self-tapping screws with 2.7mm diameter were long enough to establish a bi-cortical support. The screws were screwed into the bone with a torque-screwdriver a constant torque of 0.2Nm. The same rosettes were analyzed before and after implantation and the mandible displacement two. The experimental results for the mandibular ramus present a linear behavior up to 300N load in condyle, with the Biomet implant influencing strain distribution; the maximum influence was near the implant (rosette #4) is around 59%. The average vertical displacement of the mandibular ramus (300N) was measured by machine: 1.18 (±0.02) mm for the intact mandibular ramus and 1.21 (±0.02) mm for the implanted one, which represents a 2.8% differences between the experimental models and reduce of stiffness. The maximum principal strain deformation was observed in the rosette #3 with 1360µε more 20% than the intact mandible for 300N of reaction. The experimental results show that the Biomet TMJ mandibular ramus implant changes the load transfer in the ramus, compared to the intact, with its strain shielding effect. The results indicate the minimum number of screws is three to guarantee a good load transfer but the surface preparation of condyle presents an important factor.
