Effects of experimental unilateral condylectomy followed by altered mandibular function on the maxilla and zygoma

Correction of rabbit model with mandibular ramus shortening by distraction osteogenesis at condylar neck

PURPOSE

The rabbit model has been established to mimic the effect of temporomandibular joint (TMJ) arthroplasty of ankylosis, and distraction at the level of the condylar neck is used to elongate the ascending ramus. The histomorphologic changes of TMJ and distraction gap were investigated.

MATERIALS AND METHODS

The unilateral condyles and articular discs were extirpated, and the experimental mandibular rami were shortened by 5 mm. An embedded distracter was used to restore the height of the mandibular ramus by unilateral condylar neck distraction (0.8 mm daily for 7 days). A total of 12 adult white rabbits were used, 8 in the experimental group and 4 in the control group. Of the 8 rabbits in the experimental group, 4 each were killed at 4 and 8 weeks after completion of distraction. The TMJ and distracted calluses were harvested and processed for radiographic and histologic examination.

RESULTS

An open bite was seen in all rabbits postoperatively that had diminished at the end of distraction. The newly formed condyles radiologically showed remodeling, flattening, and sclerosis. The bony transport disc had gradually remodeled to a new condyle that was similar to the original condyle in appearance and structure. The surface of the transport disc was covered with a fibrous tissue. Moreover, the bony regeneration was perfect in the distraction gap.

CONCLUSION

These results suggest that distraction osteogenesis at the condylar neck using the traditional preauricular approach of TMJ surgery, without the additional incision, can be performed concurrently with arthroplasty of TMJ ankylosis at the same region.

Conseqüências da remoção do côndilo mandibular no crescimento da maxila e da mandíbula. Estudo experimental em ratos

Alterações de crescimento mandibular decorrentes de traumas à articulação temporomandibular são freqüentes. Entretanto, seu mecanismo não é completamente esclarecido. Assim, mediante modelo experimental, analisou-se as conseqüências da remoção do côndilo mandibular no crescimento da maxila e da mandíbula. Foram utilizados 40 Rattus norvegicus , linhagem Wistar, com um mês de idade, distribuídos em três grupos: experimental, controle-operado e controle. Sob anestesia geral, no primeiro grupo foi removido o côndilo mandibular direito, no segundo foi feito acesso cirúrgico e no terceiro nenhum procedimento foi realizado. Os animais foram sacrificados aos três meses de idade e submetidos à incidências radiográficas axial e rostro-caudal do crânio fixado. A seguir, foi realizada dissecção, e obtidas a incidência radiográfica axial do crânio e lateral das hemi-mandíbulas. A partir destas, foram feitas mensurações cefalométricas por meio de um sistema de computador. A análise estatística mostrou diferença altamente significante a menor para o desvio da linha média mandibular e para o comprimento da mandíbula no grupo experimental, bem como diferença significante a menor na altura do ramo mandibular e no comprimento da maxila. Concluiu-se que a condilectomia na fase de crescimento levou a uma assimetria significante na mandíbula, havendo também alterações significantes no comprimento da maxila.

Improved clinical use of Twin-block and Herbst as a result of radiating viscoelastic tissue forces on the condyle and fossa in treatment and long-term retention: growth relativity

Understanding mechanisms of action for orthopedic appliances is critical for orthodontists who hope to treat and retain the achieved corrections in patients with initial Class II mandibular retrognathism. That knowledge can help orthodontists produce clinically significant bone formation and avoid compression at the condyle-glenoid fossa region. It also assists us to understand the differences between short-term and long-term treatment results. It was previously thought that increased activity in the postural masticatory muscles was the key to promoting condyle-glenoid fossa growth. By analyzing results from several studies, we postulate that growth modification is associated with decreased activity, which leads to our nonmuscular hypothesis. This premise has its foundation on 3 key specific findings: significant glenoid fossa bone formation occurs during treatment that includes mandibular displacement; glenoid fossa modification is a result of the stretch forces of the retrodiskal tissues, capsule, and altered flow of viscous synovium; observations that glenoid fossa bone formation takes place a distance from the soft tissue attachment. The latter observation is explained by transduction or referral of forces. Evidence is presented, therefore, that the 3 trigger switches for glenoid fossa growth can similarly initiate short-term condylar growth modifications because the 2 structures are contiguous. These are displacement, several direct viscoelastic connections, and transduction of forces. Histologic evidence further shows that stretched retrodiskal tissues also insert directly into the condylar head's fibrocartilaginous layer. The impact of the viscoelastic tissues may be highly significant and should be considered along with the standard skeletal, dental, neuromuscular, and age factors that influence condyle-glenoid fossa growth with orthopedic advancement. These biodynamic factors are also capable of reversing effects of treatment on mandibular growth direction, size, and morphology. Relapse occurs as a result of release of the condyle and ensuing compression against the newly proliferated retrodiskal tissues together with the reactivation of muscle activity. To describe condyle-glenoid fossa growth modification, an analogy is made to a light bulb on a dimmer switch. The condyle illuminates in treatment, dims down in the retention period, to near base levels over the long-term.