J.-P. van Loon

Animal models for tracheal research.

Tracheal research covers two main areas of interest: tracheal reconstruction and tracheal fixation. Tracheal reconstructions are aimed at rearranging or replacing parts of the tracheal tissue using implantation and transplantation techniques. The indications for tracheal reconstruction are numerous: obstructing tracheal tumors, trauma, post-intubation tissue reactions, etc. Although in the past years much progress has been made, none of the new developed techniques have resulted in clinical application at large scale. Tissue engineering is believed to be the technique to provide a solution for reconstruction of tracheal defects. Although developing functional tracheal tissue from different cultured cell types is still a challenge. Tracheal fixation research is relatively new in the field and concentrates on solving fixation-related problems for laryngectomized patients. In prosthetic voice rehabilitation tracheo-esophageal silicon rubber speech valves and tracheostoma valves are used. This is often accompanied by many complications. The animal models used for tracheal research vary widely and in most publications proper scientific arguments for animal selection are never mentioned. It showed that the choice on animal models is a multi-factorial process in which non-scientific arguments tend to play a key role. The aim of this study is to provide biomaterials scientists with information about tracheal research and the animal models used.

A three-dimensional study of bending and torsion moments for different fracture sites in the mandible: an in vitro study

The aim of the study was to determine and compare bending and torsion moments across mandibular fractures, for different positions of the bite point and different sites of the fracture. Three identical resin mandibles, each with a single fracture, were used. The fracture sites were in the angle, body and symphyseal regions. A polyethylene bone plate was used for fixation. Simulated bite forces were applied at 13 bite points. For each bite point, the displacements of the fragments were registered and converted into bending and torsion moments across the fracture. Positive bending moments were defined as those moments that caused compression at the lower border and tension at the alveolar side of the mandible; negative bending moments did the opposite. Angle fractures had relatively high positive bending moments. Body fractures had positive as well as negative bending moments and the highest torsion moments. Symphyseal fractures had negative bending moments only and relatively high torsion moments. It was found that angle, body and symphyseal fractures each have a characteristic load pattern. These load patterns should play a decisive role in the treatment of mandibular fractures with regard to number and positioning of plates.

A three-dimensional study of loads across the fracture for different fracture sites of the mandible

The loads across the fracture depend on variables such as position of the fracture and the bite point. Up to now, no study has described systematically the influence of these two variables on these loads. The aim of this study was to describe and compare value and direction of the loads across the fracture for different positions of fractures in the mandible. In a three-dimensional model, bending and torsion moments and shear forces were compared for five mandibular fractures. The fractures were located in, respectively, the angle, posterior body, anterior body, canine and symphysis region. Positive bending moments were defined to give compression at the border, negative bending moments to give compression at the alveolar side of the mandible. The angle and posterior body fracture have high positive bending moments, small torsion moments and high shear forces. The anterior body, canine and symphysis fracture have high negative bending moments and high torsion moments with similar maximum values. The number of bite points with negative bending moments were different for all fractures. These bite points were always located on the fractured side. It is concluded that mandibular fractures can be divided roughly into two groups with similar load patterns across the fracture. One group consists of angle and posterior body fractures, the other group consists of anterior body, canine and symphysis fractures.

The Theoretical Optimal Center of Rotation for a Temporomandibular Joint Prosthesis: A Three-dimensional Kinematic Study

A unilateral temporomandibular joint (TMJ) prosthesis may cause dysfunction of the contralateral, natural TMJ because of lack of translatorial movements of the prosthetic side. The natural translatorial capacity of the mandible can be restored in part by a TMJ prosthesis with a fixed center of rotation (CR), positioned inferiorly to the center of the natural mandibular condyle. The aim of this study was to determine the optimal position for the fixed CR of a unilateral TMJ prosthesis. A mathematical model was used to analyze different positions of the CR. These positions were evaluated based on the calculated rotation of the mandible in the frontal (θf) and horizontal (θh) plane, and the mediolateral movement (MLM) of the contralateral natural condyle. For current TMJ prostheses, with the CR positioned in the center of the natural condyle, 6h exceeded the natural limits. When the CR was shifted inferiorly, all parameters improved, particularly θh. The addition of an anterior shift to an inferior shift slightly worsened θf, while the addition of a posterior shift to an inferior shift slightly improved θf and worsened MLM. We concluded that the functioning of the contralateral TMJ improves by shifting the CR inferiorly. An anterior shift may be added to remain within the contour of the mandibular ramus. The proposed position of the CR is 15 mm inferior to the center of the natural condyle, combined, if necessary, with an additional anterior shift of 5 mm.

Wear-testing of a temporomandibular joint prosthesis : UHMWPE and PTFE against a metal ball, in water and in serum

For a temporomandibular joint prosthesis, an estimation of the wear rate was needed, prior to patient application. Therefore, we determined the in vitro wear rate of the ball-socket articulation of this prosthesis, consisting of a metal head and an ultra-high molecular weight polyethylene (UHMWPE) cup. The basic testing configuration consisted of one 8-mm diameter stainless-steel ball, rotating between two conforming cups with a minimum thickness of 5 mm. For validation of the testing apparatus, two cup materials, in two lubricants, were tested. Both cup materials, UHMWPE and polytetrafluoroethylene (PTFE) were tested in deionized water, as well as in a serum-based solution. For UHMWPE in serum, eight samples were tested, for the other combinations four samples. For UHMWPE, the tests ran for 7 million cycles, for PTFE between 0.8 and 1.7 million cycles. For UHMWPE, the wear rate was 0.006 and 0.47 (mm3/10(6) cycles), in water and in serum, respectively. For PTFE, the wear rate was 2.8 and 47 (mm3/10(6) cycles), in water and in serum, respectively. For reason that testing in serum simulates the in vivo situation best, it was concluded that the wear rate of the TMJ prosthesis articulation is 0.47 (mm3/10(6) cycles), which is considered acceptable.

Loading of a Unilateral Temporomandibular Joint Prosthesis: A Three-dimensional Mathematical Study

The load on the prosthetic side and the influence of the design on the remaining natural contralateral TMJ must be known before a unilateral temporomandibular joint (TMJ) prosthesis can be developed. The aim of the present study was to determine the maximum loading of the TMJ prosthetic side and the natural contralateral TMJ and to investigate the influence of the location of the center of rotation of the prosthesis on the maximum loading. For this purpose, a mathematical model of the mandible with a unilateral TMJ prosthesis with a fixed center of rotation (CR) was developed. The location of the CR of the TMJ prosthesis was varied from the middle of the natural mandibular condyle to 15 mm inferior to this location. Although the maximum joint reaction forces changed as a result of a unilateral TMJ prosthesis, the trend of the loading curves was similar to that of an intact mandible. A unilateral TMJ prosthesis resulted in a 50% higher loading of the prosthetic side, while the load on the natural contralateral TMJ remained within normal limits. The maximum load on the prosthetic side occurred during molar bites and could reach 100 N in the cranial direction, 30 N in the ventral direction, and 25 N in the medio-lateral direction. The location of the CR did not have a significant influence on the loading of the TMJ prosthesis and the natural contralateral TMJ. Key words: temporomandibular joint, temporomandibular joint prosthesis, models, biomechanics, bite force, computer simulation.

A short-term study in sheep with the Groningen temporomandibular joint prosthesis

As part of the pre-clinical testing process of a newly developed temporomandibular joint (TMJ) prosthesis, animal experiments were performed. In 14 sheep, the right TMJ was replaced by the developed TMJ prosthesis. The prosthesis consisted of a skull part, a mandibular part and an intervening polyethylene disc. In the first series (6 sheep), three designs were tested, differing in the applied metal (stainless steel or titanium) and in the fitting method of the skull part (a fitting member or bone cement). The sheep were sacrificed after 8-16 weeks. In the second series (8 sheep), the preferred titanium fitting member design was applied, and the sheep were sacrificed after 2-10 weeks. One sheep was excluded because no correct position of the prosthesis parts could be achieved. At sacrifice, the removal torque of the screws was measured, and the surrounding tissues were harvested for histologic examination. The sheep recovered well and functioned until the end of the scheduled sacrifice date. Encountered problems were two disc dislocations, one fistula formation, and one screw failure. All mandibular parts were clinically stable, as were most skull parts with a fitting member, and one of both skull parts fitted with bone cement. The clinically observed stability was confirmed by the removal torque values, which indicated well-integrated screws. It is concluded that the TMJ prosthesis could remain stable and functional over the initial healing period. The main restriction of the sheep model is the much larger translatory capacity compared with patients, which adversely influences tissue healing.

In vivo and in vitro experience with the PUCA-II, a single-valved pulsatile catheter-pump

The Pulsatile Catheter (PUCA) pump is a trans-arterial pulsatile ventricular assist device that can be used for short-term left ventricular support. The separate inflow and outflow valves in the first version of the device (PUCA-I) were replaced by a single inflow/outflow valve in the latest PUCA pump version (PUCA-II). The new combined valve was tested during in vitro (mock circulation) and in vivo experiments for valve leakage, flow resistance, and thrombus formation. During the in vitro experiments a maximum valve leakage of 6% during ejection and 21% during aspiration was found. The maximum flow resistance coefficient (K) was 4. The animal experiments demonstrated that the PUCA-II could be positioned within a few minutes into the left ventricle without X-ray guidance and without using a vascular graft. Thrombi were not found in the combined valve after total pump time of 3 hours, which proved the good washout of the valve. Initial experiments to position the pump in the right ventricle through the pulmonary artery were successful and contributed to the development of a new application for the device.

The development of a valveless cardiac assist device attached to the ventricular apex.

Konstantinov et al, in October, 1991, published a novel way to bridge a patient for heart transplantation. They proposed to cut off both ventricles high under the atrioventricular groove, leaving the atria, aorta, and pulmonary artery and their valves intact and to attach pneumatically driven, valveless pulsating pouches to assist the heart until a donor could be found. The removal of the ventricles just below the atrioventricular groove is called the “high cut”; it, however, destroys the chordae tendineae rendering the mitral and tricuspid valves insufficient. These have to be replaced by tissue inflow valves. We chose to cut off the ventricles at a lower level (the “low cut”) to leave the papillary muscles on both sides intact, thereby saving the integrity of the mitral and tricuspid valves. Pulsating pouches were made to fit the heart at this lower level. They can be easily connected to the remaining heart after a specially designed cuff has been sutured over the ventricular stumps. The pouches were pumped during the systole of the natural heart, but the myocardium may have to be electrically stimulated during systole to prevent undue distension. If the turgor is too weak to prevent distension, a sleeve over the ventricles is provided. To find the best location for these pouches, human cadaver implantations were done and the pre peritoneal cavity was found to be the most suitable. In vitro testing to determine how much flow could be pumped was done by attaching the pouches to fresh pig hearts and connecting them to a double sided mock circulation. A flow of more than 4 L/min was obtained, proving that all four valves could be saved. Our pouches are designed to be used as a bridge to transplantation for those patients whose valves are still fully operational. However, the application of these pouches might become much broader if this technology could be part of a permanent artificial heart.