VOL. 2, No. 2, FEBRUARY 2013 26  TRAUMA Refixation of osteochondral fractures by ultrasound-activated, resorbable pins AN OVINE IN VIVO STUDY H. Neumann, A. P. Schulz, J. Gille, M. Klinger, C. Jürgens, N. Reimers, B. Kienast From Department of Trauma & Orthopaedic Surgery, University of Lübeck, Lübeck, Germany  H. Neumann, MD, Trauma & Orthopaedic Surgeon, Associate Specialist  C. Jürgens, MD, PhD, Professor, Trauma & Orthopaedic Surgeon, Director  B. Kienast, MD, Trauma & Orthopaedic Surgeon, Consultant Berufsgenossenschaftliches Unfallkrankenhaus Hamburg, Department of Traumatology, Orthopaedics and Sports Medicine, Bergedorfer Straße 10, 21033 Hamburg, Germany.  A. P. Schulz, PhD, MD, MRCS(Glasgow), Trauma & Orthopaedic Surgeon, Consultant, Head of Department Universitätsklinikum SH - Campus Lübeck, Biomechanical Laboratory and Orthopaedic Research, Ratzeburger Allee 160, 23538 Lübeck, Germany.  J. Gille, MD, PhD, Trauma & Orthopaedic Surgeon, Consultant Universitätsklinikum SH - Campus Lübeck, Department of Traumatology & Orthopaedics, Ratzeburger Allee 160, 23538 Lübeck, Germany.  M. Klinger, MD, PhD, Anatomist Universitätsklinikum SH - Campus Lübeck, Institute for Anatomy, Ratzeburger Allee 160, 23538 Lübeck, Germany.  N. Reimers, Master of Business Engineering (MBE), Senior Manager Medical Research Stryker Trauma GmbH, Prof.- Kuentscher-Str. 1-5, 24232 Schoenkirchen/Kiel, Germany. Correspondence should be sent to Dr H. Neumann; e-mail: hanjo_neumann@gmx.de 10.1302/2046-3758.22.2000099 $2.00 Bone Joint Res 2013;2:26–32. Received 31 May 2012; Accepted after revision 17 December 2012 Objectives Osteochondral injuries, if not treated adequately, often lead to severe osteoarthritis. Possible treatment options include refixation of the fragment or replacement therapies such as Pridie drilling, microfracture or osteochondral grafts, all of which have certain disadvantages. Only refixation of the fragment can produce a smooth and resilient joint surface. The aim of this study was the evaluation of an ultrasound-activated bioresorbable pin for the refixation of osteochondral fragments under physiological conditions. Methods In 16 Merino sheep, specific osteochondral fragments of the medial femoral condyle were produced and refixed with one of conventional bioresorbable pins, titanium screws or ultrasound-activated pins. Macro- and microscopic scoring was undertaken after three months. Results The healing ratio with ultrasound-activated pins was higher than with conventional pins. No negative heat effect on cartilage has been shown. Conclusion As the material is bioresorbable, no further surgery is required to remove the implant. MRI imaging is not compromised, as it is with implanted screws. The use of bioresorbable pins using ultrasound is a promising technology for the refixation of osteochondral fractures. Article focus  Can ultrasound-activated, resorbable pins refixate osteochondral fractures suffi- cient under physiological conditions?  Is there any effect on the surrounding car- tilage by the induced temperature of the SonicPin system (Stryker GmbH)?  Is there an advantage to ultrasound acti- vation compared with conventional pins? Key messages  The SonicPin system can refixate osteo- chondral fractures in an ovine in vivo study  The healing ratio is higher than the con- ventional resorbable pins, but lower than the screw fixation  No cartilage damage could be observed by the application of the SonicPin system Strengths and limitations  The ovine in vivo study concept can simu- late physiological conditions  First examination of ultrasound-activated pins on osteochondral defect repairs  Full weightbearing might have a negative influence on healing ratio (no significant difference concerning resorbable implants) Introduction Osteochondral injuries, if not treated ade- quately, often lead to severe osteoarthritis of the affected joint. 1 Osteochondral fractures, some- times called ‘flakes’, occur as a completely detached or partially attached fragment. With- out refixation of the osteochondral fragment, human cartilage can only repair these defects imperfectly. During healing of chondral defects, fibrous tissue is formed, which has inferior char- acteristics compared with intact cartilage in terms of weightbearing and surface structure. 2 Most osteochondral injuries occur in knee lesions, especially if the patella is dislocated. 3 Often, a fragment of the lateral condyle or a part of the rear side of the patella is detached. 4 Freely available online Keywords: Osteochondral fracture, Ultrasound-activated pin, Bioresorbable implant, Sheep study, Cartilage, Polylactide