Total alloplastic temporomandibular joint (TMJ) reconstruction is a reliable treatment modality in patients with severely diseased TMJ with good clinical behaviour. TMJ mandibular function after alloplastic reconstruction has scarcely been analysed as a biomechanical parameter and investigation has generally been limited to interincisal measurements without deeper insight into joint kinematics. Dynamic stereometry to assess condylar movements relative to the fossa was performed at the 5 year follow-up of a patient who underwent condylar resection of the right TMJ followed by total alloplastic joint reconstruction to treat pigmented villonodular synovitis. The patient could achieve wide mouth opening, but overall mandibular kinematics showed a strong deviation towards the prosthetic side due to the lack of mandibular translation caused by the absence of the lateral pterygoid attachment. Possible overloading of the joint contralateral to the TMJ prosthesis might be prevented by optimizing replacement joint design.
Total alloplastic reconstruction of the temporomandibular joint (TMJ) is acknowledged as a safe, effective and reliable treatment modality in patients with advanced TMJ disease. Over time, total joint reconstructions (TJRs) have shown reliable clinical behaviour in subjects operated on to treat inflammatory arthritis, recurrent fibrosis and/or ankylosis, failed tissue graft, failed alloplastic reconstruction, and loss of vertical mandibular height and/or occlusal relationship due to bone resorption, trauma, developmental abnormality, or pathological lesions. Although the patients’ overall quality of life has been assessed extensively, mandibular function as a biomechanical parameter has scarcely been analysed and investigation has generally been limited to measurements of the trace of an interincisal point without a deeper insight into mandibular or joint kinematics in particular. New jaw tracking systems permit in vivo measurements of mandibular motion. Recently, a novel method called dynamic stereometry has been developed to assess condylar movements relative to the fossa. This non-invasive, real time method tracks the three-dimensional (3D) relationship between the TMJ articular surfaces for any type of movement in vivo. The position of the mandibular condyle relative to the temporal fossa is measured accurately during spontaneous movements in real time. In this paper, the authors describe the first case from an ongoing study of subjects with alloplastic TMJ replacement in which they analyse in detail the kinematics of the TMJ prosthesis and compare it to the contralateral natural joint.
Materials and methods
In June 2004 a 22-year-old woman underwent a condylectomy for the treatment of pigmented villonodular synovitis (PVNS) of the right TMJ. The condyle had been replaced by a 2.4 reconstruction plate (Synthes AG, Oberdorf, Switzerland) as a space-holder for later TMJ reconstruction. One and half years later, total joint reconstruction was performed with a TMJ Concepts™ prosthesis (TMJ Concepts Inc., Ventura, CA, USA) ( Fig. 1 A). At the 5 year follow-up after total joint replacement, clinical measurement of interincisal opening was 50 mm with good occlusal stability. The radiological follow-up at 5 years showed no heterotopic bone formation and no recurrence of PVNS-associated lesions ( Fig. 1 B).
Dynamic stereometry consists in combining data originating from a non-invasive tomographic imaging system with jaw tracking in order to determine the position and motion of the mandible and in particular of the entire condyle within the fossa in 3D. The method has been in use for over a decade and has provided normative data on mandibular biomechanics in asymptomatic subjects. Briefly, coronal X-ray image stacks with 0.4 mm × 0.4 mm × 0.4 mm voxels are taken using a DVT scanner (KaVo 3D eXam ® , KaVo GmbH, Leutkirch, Germany) with the subject biting into a reference custom-made occlusal splint for the combination of imaging with jaw tracking data. Jaw tracking is performed by three linear cameras with fixed geometry detecting the Cartesian coordinates of light emitting diodes (LEDs) arranged on the vertices of triangular target frames. The target frames are rigidly connected to the teeth or to dental implants. Sampling frequency is 200 Hz and geometrical resolution better than 5 μm. TMJ reconstruction and animation are performed on a Windows™ PC. Segmentation of magnetic resonance (MR) scans yields software models of the rigid anatomical structures. These are animated by means of mathematical transformations applied continuously over time. Error analyses show that the greatest geometric distortion <0.9% occurs craniocaudally and reconstruction errors are ≤1 mm.
For kinematic recording, one target frame was fixed to the upper premolars, one to the lower premolars. Target frame attachments did not interfere with each other or with occlusion. After static reference recordings, jaw tracking was performed on the left and right sides. Jaw opening/closing and protrusion were performed three times each.
Data analysis and statistics
The coordinate system used was oriented according to the sagittal plane ( X and Y axes) and to the Camper plane ( X and Z axis). Thus, the X coordinate increased in the dorsoventral direction, the Y coordinate increased in the caudocranial direction and the Z coordinate increased towards the patient’s right hand side. For each recording (i.e. jaw opening/closing and protrusive movements) the authors determined the trajectories of the lateral condylar poles and of a mandibular interincisal point. For each trajectory, the vectors between the points in maximum intercuspation (MI) and in maximum opening (MO) were determined. These two positions corresponded to the average coordinates of the lateral poles and of the interincisal point when their velocity was ≤5% than the maximum velocity of the movement.
The distance between the trajectory points at MI and MO and the curvilinear length of the trajectories were measured. Descriptive statistics of the vector components and of the distances and curvilinear lengths were calculated. ANOVA for repeated measurements at a significance level of α = 0.05 was performed for the parameters determined. The study protocol was approved by the ethics committee of the University of Basel (Ref. Nr. EK 241/2010).
Fig. 2 shows the patient’s mandible with her mouth closed, at MO in sagittal view as well as at MO in frontal view. Fig. 3 A shows an oblique frontal view of the trajectories of the right lateral condylar pole, an interincisal mandibular point and the left lateral condylar pole during one opening/closing cycle embedded in the subject’s bony anatomy. Fig. 3 B shows six trajectories of the same mandibular points superimposed during opening/closing cycles. Fig. 3 C depicts the trajectories during protrusive cycles.
The vector components between the points MI and MO for the left condylar pole, a mandibular interincisal point, and the right condylar pole are listed in Table 1 for opening/closing and protrusive movements. Table 2 shows the values of the distances between the points MI and MO and of the curvilinear lengths of the trajectories. These parameters differed significantly between the two condylar poles for both opening/closing and protrusive movement types ( p = 0.005 and p = 0.012, p = 0.038 and p = 0.018, respectively). Mandibular kinematics and joint function are available in the online version of the article .
|Left condylar pole||Mandibular interincisal point||Right condylar pole|
|Open/close||24.3 ± 5.5||−3.5 ± 2.4||−0.8 ± 0.5||−17.9 ± 3.4||−38.5 ± 4.0||7.8 ± 2.6||4.3 ± 1.8||3.1 ± 0.6||0.7 ± 0.4|
|Protrusion||12.4 ± 1.7||−4.5 ± 1.7||−1.9 ± 0.7||6.0 ± 0.9||−2.7 ± 3.1||8.8 ± 2.1||0.7 ± 0.4||0.8 ± 0.6||0.7 ± 0.2|