Intervention group subjects will receive injectable platelet rich fibrin (i-prf) . The (i-prf) will be injected in the periodontal ligament of maxillary anterior teeth during en masse retraction.The injection sites will be the sites of bone compression to target the surfaces of the bone where osteoclastogenesis and bone resorption occurs during en masse retraction.

Considering that the obtained (i-prf) after centrifuging would be 4 ml, 1 ml will be injected intraligamentally distal to the right and left canines, and 0.5 ml will be injected intraligamentally palatal to each incisor. The study group will receive i-PRF intraligamentally in the periodontal ligament space of the maxillary six anterior teeth three times as follow, just before anterior teeth retraction, and after 21 days of the retraction, and after 42 days of the retraction. Before each time of injection, an anesthetic solution will be administered for pain control.

Subjects in the control group will only receive sham (placebo) injection three successive times with and interval of 21days between each injection, similar to the timepoints of (i-prf) injection in the intervention group. Also the sites of injection will be similar to the sites of injection of the intervention group; 1 ml of the placebo agent will be injected intraligamentally distal to the right and left canines, and 0.5 ml will be injected intraligamentally palatal to each incisor. An anesthetic solution will be administered for pain control before the administration of the sham injections.

I-prf is a second-generation PRP where autologous platelets and leukocytes are present in a complex fibrin matrix and can be obtained from blood using low-speed centrifugation without adding anticoagulants.

Alginate impression will be taken for each participant to obtain a plaster model obtained at five time points: before incisor retraction (T0), after one month (T1), after two months (T2), after three months(T3), and after four months (T4) of retraction. The models will be coded with numbers assigned to the participants, and will be sent to an assessor not participating in the research. In both groups, the amount of space closure will be calculated by a digital caliper in millimeters for each time point (T) and averaged for right and left sides.

Pain will be assessed using a visual analogue scale questionnaire that will be completed by the patient starting from the day following i-prf injection in the intervention group or the sham injection in the control group, the questionnaire will be repeated every month until the fourth month