Recent experimental research demonstrated that non-reducing temporomandibular joint (TMJ) disc displacement in growing rabbits impaired mandibular growth. TMJ disc displacement is also shown to induce histological changes of the condylar cartilage. The authors hypothesized that the severity of these changes would correlate to the magnitude of mandibular growth. Bilateral non-reducing TMJ disc displacement was surgically created in 10 growing New Zealand White rabbits. Ten additional rabbits constituted a sham operated control group. Aided by tantalum implants, growth was cephalometrically determined for each mandibular side during a period equivalent to childhood and adolescence in man. At the end of the growth period, histologically classified cartilage features were correlated with the assessed ipsilateral mandibular growth. Non-reducing displacement of the TMJ disc during the growth period induced histological reactions of the condylar cartilage in the rabbit model. The severity of cartilage changes was inversely correlated to the magnitude and the direction of mandibular growth, which resulted in a retrognathic growth pattern.
The mandibular condyles represent important growth sites within the facial skeleton. Condylar growth does not set the pace of mandibular growth, but it provides regional adaptive growth of considerable clinical significance because the upward and backward directed condylar growth displaces the mandible anteriorly and inferiorly as a whole .
It has repeatedly been demonstrated in animal models that temporomandibular joint (TMJ) disc displacement induces condylar cartilage changes , with a positive correlation between the severity of disc displacement and the severity of cartilage reaction . The condylar cartilage is a biologically unique articular cartilage with an exceptional capacity for adaptive modelling in response to external stimuli . The cellular activity of the cartilage is regulated by various local growth factors and changes in the cartilage’s biophysical environment, such as altered articulating function, triggers or impairs their endogenous expression, leading to increased or decreased condylar growth .
Several clinical studies of facial asymmetry and mandibular retrognathia, have reported an association with coexisting non-reducing TMJ disc displacement in adults as well as in children and adolescents . Whether the adverse craniofacial growth predisposed for displacement of the TMJ disc or vice versa was clinically unclear, but cause and effect has been established in longitudinal experimental studies verifying that surgically induced non-reducing TMJ disc displacement with onset during the growth period in a rabbit model, caused subsequent ipsilateral impairment of mandibular growth , and that with bilateral joint affliction, the extent of growth impairment corresponded to the development of mandibular retrognathia in man .
A non-deranged TMJ articulation seems necessary for maintenance of an optimal biophysical environment for the condylar cartilage, so the authors hypothesized that the severity of cartilage reactions following TMJ disc displacement would correlate with the magnitude of mandibular growth. The aim of this study was to perform a histological evaluation of the condylar cartilage response to non-reducing TMJ disc displacement during the growth period in a rabbit model and to correlate histologically classified cartilage features with ipsilateral mandibular growth.
Materials and methods
Twenty New Zealand White rabbits ( Oryctolagus cuniculus ) were randomized into two groups: an experimental group ( n = 10) in which bilateral non-reducing TMJ disc displacement was surgically created; and a sham operated control group ( n = 10) in which the same surgical procedure was performed but without manipulation of the TMJ disc.
The animals were 10 weeks old at the beginning of the study and were allowed to grow for a mean of 96 days (range 93–98 days). The rabbit’s growth period approximated childhood and adolescence in man . A non-operated third control group was not considered ethically justified because the sham operation, as performed in this study, has previously been proven not to influence facial growth . The animals were given free access to a regular diet of pellets throughout the study. One animal in the experimental group initially had problems consuming normal food after the surgery and was given soft food for the first week postoperatively. This animal was capable of eating normally during the rest of the study period. No animal was lost during the study.
The study was approved by the Ethics committee on animal experiments, Umeå University, Sweden (Registration No. A 128-00).
Bilateral TMJ disc displacement was surgically created in each experimental animal . The approach to the TMJ was made via a skin incision posterior to the orbit. Blunt dissection of the soft tissue covering the joint was performed, and the joint area was disclosed ( Fig. 1 a) . A small part of the zygomatic arch was removed with a dental bur, providing an adequate view of the surgical area. The continuity of the zygomatic arch was maintained ( Fig. 1 b). The capsule was incised. The disc and its attachments were identified and a ligature was sutured through the anterior part of the disc. The medial, anterior, and lateral disc attachments were detached ( Fig. 1 c) and the disc was pulled anteriorly, placing the intact posterior disc attachment above the condyle. A ligature through a hole drilled in the zygomatic arch anchored the displaced disc anteriorly ( Fig. 1 d). Maintenance of the incorrect disc position was checked, and the wound was closed in layers ( Fig. 1 e).
The sham operation followed the same procedure until the disc was exposed. The wound was closed without any disc manipulation.
At study inception, tantalum spheres were inserted on the left side and tantalum pins on the right side of the mandible, to allow longitudinal cephalometric evaluation of growth of each side. Two titanium alloy screws in the calvarium served the dual purpose of reproducibly positioning the animal’s head in a specially designed cephalostat, and providing reference structures for superimposition of serial cephalograms . The tantalum implant interfered with a lower incisor in one control animal and in two experimental animals resulting in biassed growth measurements. These measurements were discarded, as were the ipsilateral joint specimens, resulting in a total number of 37 specimens with matching mandibular growth measurements.
TMJ surgery and the insertion of screws and implants was performed under general anaesthesia with 0.4 ml midazolam Dormicum ® /kg of body weight, intraperitoneally, and 0.2–0.3 ml of fentanyl, fluanison Hypnorm ® /kg of body weight, intramuscularly. Subcutaneous injections of 0.3–0.5 ml felypressin, prilokain Citanest Octapressin ® were given prior to incisions in the scalp, the TMJ area, and the alveolar mucosa to achieve local anaesthesia.
Following the surgical procedures, the animal was given 0.1 ml buprenorphine (Temgesic ® ) subcutaneously/kg of body weight for analgesia, and approximately 15 ml of saline/kg of body weight to prevent dehydration.
After 3 months, the animal was killed with an intravenous injection of approximately 1.2 ml tiopental Pentotal ® /kg of body weight.
Radiography and analysis
Lateral cephalograms were exposed at study inception and after 3 months. The cephalograms were digitized and superimpositions and measurements were conducted on a personal computer. Positions of the left and right tantalum implants were plotted digitally in each cephalogram, giving them x – and y -coordinates to the nearest tenth of a millimetre. The position of each tantalum implant in the inceptive cephalogram was defined as the implant’s baseline position (origin), and the magnitude and direction of growth after 3 months was determined for the left and right sides, respectively. This experimental cephalometric method had a proven measurement precision of 0.4 mm in longitudinal studies . The technique and the effect of surgically induced bilateral non-reducing TMJ disc displacement on overall mandibular growth in the present material have been presented previously . The present study makes use of the technique’s ability to identify growth for right and left mandibular sides, respectively, to correlate the intra-individual cartilage reactions with the subsequent amount of ipsilateral mandibular growth.
After death, the rabbit was decapitated and the skull skinned. Intermaxillary fixation with a wire maintained the intercuspal position. The skull was kept in 10% buffered formalin (pH 7.0) until block preparation. Each TMJ specimen was removed en bloc , approximately 4 × 2 × 2 cm in size. The tissue laterally over the joint was left intact.
The specimen blocks were decalcified for 2 weeks in 22% trisodium citrate buffered formic acid that was exchanged every second day. After decalcification, the specimens were dehydrated in graded alcohol for 3 days followed by incubation in methyl salicylate (HistoLab AB, Sweden) overnight at 37 °C and subsequently in a mixture of 1% celloidin in methyl salicylate at 37 °C for 3 days. The specimens were immersed in three graded methyl salicylate/paraffin mixtures followed by pure paraffin for 2 days. The paraffin embedded specimens were cut sagittally throughout the lateral into the central part of the TMJ using a scroll saw and thereafter cut in 5 μm sections that were stained with haematoxylin and eosin.
The histological sections were evaluated under light microscope and classified by two observers in consensus. The anterosuperior part of the condyle, opposing the articular tubercle, was chosen as the region of interest ( Fig. 2 ). The histological feature of each condylar cartilage was graded based on the degree of divergence from normative cartilage configuration and classified as no, minor, moderate, severe or destructive changes ( Fig. 3 ). For detailed classification criteria see Fig. 3 legend.
Mann–Whitney’s non-parametric test was used to test for differences in histological cartilage changes between experimental and control condyles. Independent-samples t -test for equality of means was used to test whether cartilage changes in the mandibular condyles were associated with altered magnitude and direction of mandibular growth. Spearman’s non-parametric correlation test was used for intra-individual correlation between the histologically classified features of the condylar cartilage and ipsilateral mandibular growth. p -Values <0.05 were considered statistically significant.
All but one control condyle and five experimental condyles had an even, convex contour (as in Fig. 2 a). The remaining 13 experimental condyles displayed modelling with loss of convexity and a flattened enlargement of the articulating surface (as in Fig. 2 b). All control condyles and all but one experimental condyle were covered with an intact layer of fibrous tissue ( Fig. 4 ).
Cartilage features differed significantly between experimental and control condyles ( p < 0.001). All 18 experimental condyles displayed cartilage changes of different grades, the majority of them (72%) classified as severe or destructive ( Fig. 4 ) and consistently appearing in areas exposed to articular loading. The majority of the control condyles (74%) had normative cartilage ( Fig. 4 ). The remaining control condyles displayed minor or moderate cartilage changes, which opposed small iatrogenic irregularities on the temporal joint component, caused by the sham surgery.
When condylar cartilage changes were present, mandibular growth on the same side was significantly reduced compared with growth when condylar cartilage was unaffected ( p = 0.025) and the horizontal growth vector was significantly shorter ( p = 0.001) ( Table 1 ). There was a significant inverse correlation between the histologically classified features of the condylar cartilage and the magnitude of ipsilateral mandibular growth ( r s = −0.444) ( p = 0.006). A strong negative correlation was seen between the cartilage features and the length of the horizontal growth vector ( r s = −0.624) ( p < 0.001). Negative correlations were also present between the degree of cartilage changes and the magnitude of ipsilateral mandibular growth ( r s = −0.482) ( p = 0.020) and between the degree of cartilage changes and the length of the horizontal growth vector ( r s = −0.557) ( p = 0.006) ( Table 2 ). The assessed growth of individual mandibular sides is accounted for in Table 1 and Fig. 5 .
|Cartilage classification||Mandibular growth||Horizontal vector||Vertical vector|
|No changes ( n = 14)||9.5||0.96||8.7||0.80||3.7||1.25|
|Minor changes ( n = 5)||9.4||0.36||8.4||0.24||4.1||0.75|
|Moderate changes ( n = 5)||9.2||0.72||7.8||1.44||4.7||1.09|
|Severe changes ( n = 12)||8.4||1.16||7.0||1.16||4.0||2.47|
|Destructive changes ( n = 1)||6.6||–||5.2||–||4.1||–|
|No cartilage changes ( n = 14)||9.5 *||0.96||8.7 ***||0.80||3.7 ns||1.25|
|Cartilage changes ( n = 23)||8.7 *||1.10||7.4 ***||1.27||4.2 ns||1.86|
|T -test Sig. (2-tailed)||0.025||0.001||0.345|