128 Orthodontics: Principles and Practice

the restoration of the root surface to its original contour; and a functional repair occurred when the exposed dentin has been covered by a thin layer of repair cementum, resulting in a defi -cient root outline. In both types, the PDL was restored to its original width.

The amount of root resorption repair increases with time. 7 – 9,142 Owman - Moll and Kurol 143 demonstrated more reparative cemen-tum in the resorption cavities after 6 and 7 weeks of retention when compared with 2 and 3 weeks of retention. The reparative process increased during the fi rst 4 weeks of retention and after 5 – 6 weeks, the process slowed down and reached a steady phase. 9 The reparative process seemed to continue for a long period of time.

Owman - Moll et al. 9 documented the amount of root resorption cavities repaired at different retention periods following 6 weeks of light buccally directed orthodontic force of 50 cN. After the fi rst week of retention, 28% of the resorption craters showed some degree of repair. The repair rose to 75% after 8 weeks of retention. Partial repair was recorded more often (17 – 31%) than functional or anatomical repair during the fi rst four weeks of retention. After 5 – 8 weeks of passive retention, functional repair dominated the repair process (33 – 40%). Resorptive areas with anatomical repair were registered six times more often after 8 weeks of retention (12%) than they were after the fi rst week of retention (2%). In a later study, Owman - Moll and Kurol 143 found 38%, 44% and 82% of resorption craters repaired after 2, 3 and 6 – 7 seven weeks of retention, respectively.

A recent micro - CT study has shown that root resorption continues for another 4 weeks after orthodontic force has ceased. 144 The reparative processes seems to be different for different levels of force application. In the study, repara-tive process reached a steady rate after 4 weeks of passive retention following the appli-cation of 4 weeks of light force whereas the majority of the reparative process occurred after 4 weeks of passive retention following the

volume of root resorption crater following intermittent orthodontic forces to be signifi -cantly lower than continuous orthodontic forces.

Extraction Versus Non - extraction Treatment Protocols There are studies which have discussed the amount of OIIRR associated with extraction treatments. 103,104,121 The approach of categoris-ing an extraction or non - extraction plan as being associated with OIIRR is overly simplis-tic. Attention should be drawn to the distance the teeth are moved. Extractions for severe crowding do not have as much impact on movement of the maxillary incisors as the dis-placement following extractions for overjet reduction.

ORTHODONTIC RELAPSE AND OIIRR Following active appliance removal, there is a conversion of the former pressure side of the active treatment period into the tension side during the relapse period. 137 Langford 8 showed that relapse forces were capable of causing sig-nifi cant root resorption for up to three months after RME.

REPAIR OF OIIRR Repair of root resorption craters begins when the applied orthodontic force is discontinued or reduced below a certain level. 5,6 According to Schwarz, 138 when the orthodontic force reduces below the optimal force of 20 – 26 g/cm 2 , root resorption stops. Many studies have demon-strated that the resorptive defects are repaired by deposition of new cementum and re - establishment of new PDL. 7,8,22,139 – 141

Henry and Weinmann 14 defi ned two types of repair: anatomical repair was characterised by