KNEE The effect of overstuffing the patellofemoral joint on the extensor retinaculum of the knee K. M. Ghosh Æ A. M. Merican Æ F. Iranpour Æ D. J. Deehan Æ Andrew A. Amis Received: 10 March 2009 / Accepted: 25 May 2009 / Published online: 13 June 2009 Ó Springer-Verlag 2009 Abstract Overstuffing the patellofemoral compartment during TKR leads to complications such as maltracking and wear, predisposing to early failure. However, there is no data describing how the patellar construct thickness affects the retinacula. This study instrumented cadaveric knees that had a Genesis II (Smith & Nephew, Memphis, TN, USA) TKR in situ. Sutures were passed along the medial patellofemoral ligament (MPFL) and the deep transverse fibre band of the lateral retinaculum, from the ilio-tibial band (ITB) to the patella. These sutures were attached to displacement transducers. Length changes in the retinacula were measured during knee flexion-exten- sion against the actions of 175 N quadriceps and 30 N ITB tensions. This was done with the natural patellar thickness, then repeated with the patella 2 mm thinner, 2 mm thicker and 4 mm thicker (overstuffed). Each thickness change caused a significant overall slackening or stretching of the MPFL (P \ 0.0001 by ANOVA), with 2.3 mm mean stretching (P \ 0.001 all angles of knee flexion by post- testing) at 4 mm thicker. The ITB-patellar band was not slackened (P = 0.491) or stretched (P = 0.346) signifi- cantly by 2 mm thickness changes. 4 mm thickening stretched the lateral retinaculum 1.1 mm (P = 0.0108). Patellar thickness affected the MPFL more than the lateral retinaculum. This difference reflected the mobile attach- ment of the lateral retinaculum to the ITB, whereas the MPFL was stretched directly between bony attachments. 2 mm overstuffing did not stretch the retinacula sufficiently to cause mechanical effects. Keywords Knee arthroplasty Á Patella retinaculum Á Patellar thickness Á Over stuffing Introduction ‘‘Over-stuffing’’ of a joint occurs during arthroplasty if an insufficient flexion, extension, or patello-femoral space is generated for the thickness of the implants inserted. When the implant has been inserted, the surrounding soft tissues will be tensed and the implant components compressed together. Overstuffing the patellofemoral compartment may lead to decreased flexion, patellofemoral maltracking, subluxation, increased patellofemoral compression and shear forces contributing to increased component wear, and possibly pain compromising the end result [1, 10, 14, 22–24, 26, 28]. An excessively thick patella may also cause practical problems for the surgeon during wound closure, because of the overstretched extensor retinaculum [17]. A thinner than normal patella increases the risk of stress fracture of the patella [15, 24]. The literature to date is in agreement regarding the importance of maintaining the pre-cut thickness of the K. M. Ghosh Á A. A. Amis (&) Department of Mechanical Engineering, Imperial College London, South Kensington Campus, London SW7-2AZ, UK e-mail: a.amis@imperial.ac.uk K. M. Ghosh Á D. J. Deehan Orthopaedic Surgery Department, Newcastle University Hospital, Newcastle upon Tyne NE2-4HH, UK A. M. Merican Á F. Iranpour Á A. A. Amis Musculoskeletal Surgery Department, Imperial College London, Charing Cross Hospital, London W6-8RF, UK A. M. Merican University of Malaya Medical Centre, Kuala Lumpur, Malaysia 123 Knee Surg Sports Traumatol Arthrosc (2009) 17:1211–1216 DOI 10.1007/s00167-009-0830-0