Abstract
The essential life functions of mastication, speech, airway support and deglutition are supported by temporomandibular joint (TMJ) function and form. Over a lifetime, this puts the TMJ complex under more cyclical loading and unloading than any other joint. Therefore, to provide long-term effective outcomes, the TMJ total joint replacement (TJR) device selected must be capable of managing the anatomical, functional and aesthetic discrepancies that dictated its use. The primary goal of TMJ TJR is the restoration of mandibular function and form. Outcomes data confirm that any pain relief attained must be considered of only secondary benefit. Despite persistent but reduced chronic pain, increased mandibular function and form improvement have been reported, resulting in quality of life improvement for 85% of custom TMJ TJR patients studied long-term. Based on the literature and the accepted orthopaedic criteria for the development and utilisation of successful TJR devices, this paper presents a rationale for the use of custom TMJ TJR devices as a ‘fitting’ management option for end-stage TMJ disorders.
End-stage temporomandibular joint (TMJ) pathology resulting in anatomical architectural form distortion and physiological dysfunction dictates the need for total joint replacement (TJR). The complex nature of the TMJ’s functional relationship with the local anatomy and masticatory muscles and the technical requirements of implanting a replacement mean that it is unreasonable to expect the replaced joint to return to its pre-morbid, fully functional condition.
The essential life functions of mastication, speech, airway support and deglutition are supported by proper TMJ function and form. This puts the TMJ complex under more cyclical loading and unloading than any other body joint over a lifetime. Therefore, to provide long-term effective outcomes, the TMJ TJR device chosen must be capable of managing the anatomical, functional and aesthetic discrepancies that influenced its choice.
There remain a number of multiply operated, anatomically distorted, dysfunctional TMJ patients, in whom chronic neuropathic, centrally mediated, pain will always be a major component of their disability. The goal of TMJ TJR is the restoration of mandibular function and form. Outcomes data confirm that any pain relief attained must be considered of secondary benefit. 85% of custom TMJ TJR patients experienced long-term quality of life improvement as a result of increased mandibular function and form improvement, despite enduring reduced chronic pain.
The surgeon should review the pertinent literature and use the TMJ TJR system that best meets the functional and form needs of each patient, based on reported long-term outcomes.
Based on the available refereed and edited literature, this paper will present the well-accepted orthopaedic criteria for the development and utilisation of successful TJR devices to establish a rationale for the use of custom TMJ TJR devices in the long-term management of end-stage TMJ disorders.
TMJ TJR devices
There are two categories of TMJ TJR devices approved by the Food and Drug Administration for implantation in the United States (Figs 1 and 2 ). First, stock (off-the-shelf) devices which the surgeon has to ‘make fit’ at implantation (Biomet Microfixation TMJ Replacement System, Jacksonville, FL, USA; TMJ Medical, Golden, CO, USA). Second, custom (patient-fitted) devices which are ‘made to fit’ each specific case (TMJ Concepts Patient-Fitted Total TMJ Replacement System, Ventura, CA, USA; TMJ Medical, Golden, CO, USA).
To date, there is only one report in the literature that compares a stock and a custom TMJ TJR system. The authors concluded that patients implanted with the custom TMJ TJR system had statistically significant better outcomes in both the subjective and objective domains than did those implanted with the stock system devices studied.
Established criteria for successful alloplastic TJR devices
After years of use, orthopaedic surgeons developed accepted criteria for successful TJR device utilisation ( Table 1 ). Applying these well-accepted criteria to TMJ TJR long-term successful utilisation, a rationale can be established for the use of custom TMJ TJR devices.
| • The components of any TJR device must be stable in situ at implantation |
| • The materials from which TJR devices are manufactured must be biocompatible and able to withstand the forces of mandibular function |
| • TJR devices must be designed to withstand the loads delivered over the full range of function of the joint to be replaced |
| • The implantation surgery must be performed for the proper indications and aseptically |
The components of any TJR device must be stable in situ at implantation
All implanted alloplastic devices depend on the principle of fixation component biointegration (screws in the case of TMJ devices) for their stability and longevity. Biointegration implies the direct incorporation of the fixation components by bone without the preliminary phase of fibrous tissue ingrowth. The requirements for biointegration are essentially the same as for primary fracture healing, basically the transmission of forces from the implant to the bone and vice versa must occur without relative motion or without intermittent loading. To assure long-term success, the most important principle in TMJ TJR must include the primary stability of the components at implantation.
The need for custom components in orthopaedic TJR is uncommon. The bony anatomy of the pelvis, femur and tibia afford the use of modular stock components that can be stabilized initially with screws, press-fitting or cementation. The bony anatomy of the mandibular ramus and the temporal glenoid fossa do not provide such options for TMJ TJR. Therefore, all TMJ TJR devices must utilize screw fixation for initial fixation and stabilisation of both the fossa and ramus/condyle components.
Compounding the anatomical and stability issues is the fact that most patients presenting with indications for TMJ TJR have deformed local bony anatomy. This may be the result of numerous failed prior surgical interventions, failed materials, as well as systemic primary or secondary end-stage disease pathology. Attempting to make stock TMJ TJR components fit and remain stable in these situations confronts the surgeon with a difficult challenge ( Fig. 3 ).
At implantation, to make stock TMJ TJR components fit, it is often the case that precious host bone must be sacrificed to create stable component-to-host-bone contact. To achieve a fit in complex cases, the surgeon may have to consider bending a stock component or shimming it with autogenous bone, bone substitute or alloplastic cement. These tactics can lead to component or shim material fatigue and/or overload fostering early failure under repeated cyclical functional loading.
Of more concern is the potential for the development of micromotion of any altered or shimmed component. Micromotion interferes with screw fixation biointegration which is necessary for component stability. Micromotion leads to the formation of a fibrous connective tissue interface between the altered component and the host bone. This can result in early loosening of the screw fixation leading to component mobility and potential early catastrophic or certain later premature device failure ( Fig. 4 ).
Custom TMJ TJR components are designed and manufactured to each patient’s specific anatomical condition on a stereolaser (SL) model developed from a protocol computed tomography (CT) scan. Therefore, the fossa and ramus components can be designed and manufactured to conform to any unique or complex anatomical host bone situation. At implantation, neither the custom TMJ TJR components nor the host bone require alteration or shimming to achieve initial component screw fixation and stability. The screw fixation secures the components intimately to the host bone mitigating the potential for micromotion and maximising the opportunity for fixation screw biointegration.
The materials from which TJR devices are manufactured must be biocompatible and able to withstand the forces of mandibular function
In 1960, Sir John Charnley reported the use of a total alloplastic prosthetic hip replacement system. He developed a metal-backed polyethylene polymer acetabular cup which articulated with a stainless steel femoral head component that was cemented in place with polymethylmethacrylate.
Modifications of this device using titanium (Ti), titanium alloy (Ti–6Al–4V), cobalt–chromium–molybdenum (Co–Cr–Mb) and ultrahigh molecular weight polyethylene (UHMWPE) have led to these materials becoming the gold standard for low friction orthopaedic TJRs. Acceptance of this management option for end-stage joint disease has made the modern practice of orthopaedic surgery impossible without the availability of TJR devices.
Employing the most advantageous physical characteristics of biocompatible materials is an essential consideration in the design and manufacture of any TJR device. The material composition of the TMJ TJR devices presently available can be found in Figs 1 and 2 .
Wrought, unalloyed titanium was originally chosen for endosteal implants and bone plates because of the rapid reaction of elemental titanium with oxygen in the air to form a thin (<10 μm) chemically inert titanium oxide layer. This layer provides a favourable surface for biointegration of implant components with bone. Titanium also has properties of strength, corrosion resistance, ductility, and machinability. The extensive literature demonstrating its biocompatibility and biointegration make titanium the metal of choice for the manufacture of the major components of TJR devices to date.
Wrought Co–Cr–Mb with its relatively high carbon content contributes to its strength, polishability, and biocompatibility. Its excellent wear characteristics when articulated against an UHMWPE presently make it the standard for the non-moveable articulating surface of most orthopaedic TJR devices.
Cast Cr–Co, often employed in the manufacture of stock TMJ TJR devices, is physically inferior to any wrought alloy. Metallurgical flaws such as inclusions and porosity found in cast Cr–Co components have been associated with the fatigue failure of metal-on-metal prostheses. These flaws may also lead to the failure of Cr–Co TJR components resulting in noxious metallic debris (metalosis) found in adjacent tissues ( Fig. 5 ).
UHMWPE is a linear unbranched polyethylene chain with a molecular weight of more than one million. Testing over four decades of use in orthopaedic TJR has led to the conclusion that UHMWPE is considered to have excellent wear and fatigue resistance for a polymeric material. To date, no cases of UHMWPE particulation-related osteolysis have been reported in the TMJ TJR literature.
TJR devices must be designed to withstand the loads delivered over the full range of function of the joint to be replaced
An important advantage afforded by a custom TMJ TJR is that the components can be specifically designed to manage the loads posed in the face of unique anatomic situations. For example, the centre of rotation of the condyle of a custom TMJ TJR can be moved vertically to accommodate closure of the open bite deformity; or the ramus component can be shaped to accommodate the amount of available mandibular host bone. This ability to vary the design to cope with the existing anatomy leads to a more predictable result in any complex clinical situation.
Custom TMJ TJR design from anatomically accurate SL models will maximize screw fixation position options for initial component stability. The positions of the screw holes can be designed to avoid the inferior alveolar canal, thereby eliminating potential injury to its contents during fixation.
Proper bicortical screw length can be predetermined and prescribed. This eliminates time consuming and frustrating intra-operative screw hole ‘probing’ to determine the appropriate fixation screw length ( Fig. 6 ). Knowing the proper screw length eliminates the potential for placing screws that are too long, which may be the cause of functional pain. In the case of the fossa component, if the sharp tips of the fixation screws penetrate beyond the medial cortex of the zygoma they can irritate the temporalis muscle. In the case of the mandibular component, too long screw tip impingement on the medial pterygoid is the concern.
The implantation surgery must be performed for the proper indications and aseptically
As with any surgical technique, outcomes are only predictable when the procedure chosen is performed correctly and aseptically, for the proper diagnosis, at the appropriate time, for the right patient, and with the right equipment.
Schmalzried and Brown report that the major causes of orthopaedic TJR failures are the result of failure of the surgeon’s implantation technique or the limitations of the device implanted to properly manage the posed anatomical situation. A custom TMJ TJR device mitigates both issues.
Established criteria for successful alloplastic TJR devices
After years of use, orthopaedic surgeons developed accepted criteria for successful TJR device utilisation ( Table 1 ). Applying these well-accepted criteria to TMJ TJR long-term successful utilisation, a rationale can be established for the use of custom TMJ TJR devices.
