Proliferation of synovial Mesenchymal Stromal/Stem Cells (MSCs) leads to synovial hyperplasia (SH) following Joint Surface Injury (JSI). Uncontrolled Yap activity causes tissue overgrowth due to modulation of MSC proliferation. We hypothesised that YAP plays a role in SH following JSI. A spatiotemporal analysis of Yap expression was performed using the JSI model in C57Bl/6 mice. Synovial samples from patients were similarly analysed. Gdf5-Cre;Yap1fl/fl;Tom mice were created to determine the effect YAP1 knockout in Gdf5 lineage cells on SH after JSI. In patients, Yap expression was upregulated in activated synovium, including a subset of CD55 positive fibroblast-like synoviocytes in the synovial lining (SL). Cells staining positive for the proliferation marker Ki67 expressed active YAP. In mice, Yap was highly expressed in injured knee joint synovium compared to controls. Yap mRNA levels at 2 (p<0.05) and 8 days (p<0.001) after injury were increased. Conditional Yap1 knockout in Gdf5 progeny cells prevented hyperplasia of synovial lining (SL) after JSI. Cellularity was significantly decreased in the SL but not in the sub-lining of injured Yap1 knockout- compared to control mice. The percentage of cells in synovium that were Tom+ increased in response to JSI in control and haplo-insufficient but not in YAP1 knockout mice (p<0.05). Modulation of YAP and proliferation of MSCs in the synovium after JSI provides a system to study the role of SH after trauma in re-establishing joint homeostasis and is a potential novel therapeutic target for the treatment of post traumatic OA

Lateral Epicondylitis is a common condition caused by angiofibroblastic hyperplasia of the tendinous insertion. Its treatment is varied and includes rest, physiotherapy, corticosteroid injection and surgical release. Of late, the role of Platelet Rich Plasma (PRP) injections have been explored, with positive results. We prospectively assessed the outcomes of lateral epicondylitis treated with PRP injections and compared the outcomes of ultrasound guided and ‘blind’ injection. This was a single centre prospective cohort study. Patient were assessed with the Disabilities of the Arm, Shoulder and Hand (DASH) Score. PRP was injected into and around the common extensor origin either with or without ultrasound guidance. The primary outcome measure was DASH score at 3 months. 45 (23F:22M) patients were recruited. The mean age was 50 years (range 35 to 79). The mean duration of follow-up was 106 days. The average pre-injection DASH score for both groups combined was 45.5 (11.7–87.5). The mean DASH score at 3 months follow-up was 27.7. The ultrasound guided injection group had a mean improvement in DASH score from 45.5 to 31.2. The “blind” injection group had a mean improvement in DASH from 44.4 to 27.7. No complications were noted. There was no statistically significant difference in improvement in DASH score between the two groups. Our study provides further evidence to support of the use of PRP injections in the treatment of epicondylitis of the elbow. Interestingly, however, we found that the use of Ultrasound guidance does not appear to improve patient outcomes