Obstructive Sleep Apnoea (OSA) is a sleep disorder characterised by upper airway collapse resulting in oxygen desaturation and sleep fragmentation. One treatment is a Mandibular Advancement Splint (MAS), a dental device which holds the lower jaw forward during sleep and increases upper airway space. However treatment success is variable with ~30% of patients experiencing no clinical benefit. Dynamic magnetic resonance imaging (MRI) shows mandibular advancement (MA) produces lateral airway expansion via direct connection between the lateral walls and mandibular ramus. A candidate for this tissue connector is the pterygomandibular raphe (PmR), an aponeurotic structure joining the buccinator and superior pharyngeal constrictor muscles, absent in 36% of people. We hypothesised that absence of PmR between pharyngeal muscles may reduce the airway response to MAS and relate to treatment failure.  Method: Retrospective analysis of MRI data of 69 OSA patients with known MAS treatment response. T1-weighted axial images (3mm slice thickness) were viewed to assess tissue appearance between airway walls and mandibular ramus as 1) continuous (no PmR) or 2) defined structure suggestive of fascial component (PmR). Two observers independently graded scans an PmR presence/absence blind to treatment outcome. Inter-observer agreement was assessed (Kappa). Categorical PmR presence/absence scores were compared between MAS Responders and Non-responders. Results: Observers agreed on classification in 68% of cases (positive observations 76%, negative observations 54%). Cohen’s Kappa was 0.31 (fair). No relationship between PmR and MAS response was found (Chi-square = 2.4, p =0.12). ~20% more PmR observations occurred in responders (non-significant). Conclusion: MRI appearance of PmR is not well described and assessment on 3mm axial slices only showed fair inter-observer agreement. Association of PmR and MAS response could not be confirmed, although imaging may be inadequate. Results suggest a weak signal for absence of PmR more often in Non-responders and further exploration is needed.