VOL. 94-B, No. 9, SEPTEMBER 2012 1241  LOWER LIMB The effect on mechanical axis deviation of femoral lengthening with an intramedullary telescopic nail R. D. Burghardt, D. Paley, S. C. Specht, J. E. Herzenberg From International Center for Limb Lengthening, Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore, Baltimore, Maryland, United States  R. D. Burghardt, MD, Orthopaedic Surgeon AKK Altonaer Kinderkrankenhaus, Abteilung für Kinderorthopädie, Bleickenallee 38, 22763 Hamburg, Germany.  D. Paley, MD, FRCSC, Medical Director, Paley Advanced Limb Lengthening Institute Paley Advanced Limb Lengthening Institute, St. Mary's Hospital, Kimmel Building, West Palm Beach, Florida 33407, USA.  S. C. Specht, MPA, Research Program Manager  J. E. Herzenberg, MD, FRCSC, Director of International Center for Limb Lengthening, Director of Pediatric Orthopedics at Sinai Hospital of Baltimore International Center for Limb Lengthening, Rubin Institute for Advanced Orthopedics, Sinai Hospital of Baltimore, 2401 West Belvedere Avenue, Baltimore, Maryland 21215, USA. Correspondence should be sent to Dr J. E. Herzenberg; e-mail: jherzenberg@lifebridgehealth.org ©2012 British Editorial Society of Bone and Joint Surgery doi:10.1302/0301-620X.94B9. 28672 $2.00 J Bone Joint Surg Br 2012;94-B:1241–5. Received 9 November 2011; Accepted after revision 26 April 2012 Internal lengthening devices in the femur lengthen along the anatomical axis, potentially creating lateral shift of the mechanical axis. We aimed to determine whether femoral lengthening along the anatomical axis has an inadvertent effect on lower limb alignment. Isolated femoral lengthening using the Intramedullary Skeletal Kinetic Distractor was performed in 27 femora in 24 patients (mean age 32 years (16 to 57)). Patients who underwent simultaneous realignment procedures or concurrent tibial lengthening, or who developed mal- or nonunion, were excluded. Pre-operative and six-month post-operative radiographs were used to measure lower limb alignment. The mean lengthening achieved was 4.4 cm (1.5 to 8.0). In 26 of 27 limbs, the mechanical axis shifted laterally by a mean of 1.0 mm/cm of lengthening (0 to 3.5). In one femur that was initially in varus, a 3 mm medial shift occurred during a lengthening of 2.2 cm. In a normally aligned limb, intramedullary lengthening along the anatomical axis of the femur results in a lateral shift of the mechanical axis by approximately 1 mm for each 1 cm of lengthening. Limb lengthening has traditionally been accomplished with the use of external fixa- tion. 1-3 In order to shorten the treatment time, lengthening over an intramedullary nail was developed. 4 More recently, implantable tele- scopic intramedullary nails have eliminated the need for external fixation during lengthening. 5-7 Bliskunov 8 devised the first practical mechani- cally activated lengthening nail during the early 1980s. His device worked by hip rota- tion, actuating a linkage from the femoral nail to the pelvis. Guichet et al 9 simplified the con- cept by allowing rotation through the length- ening callus to actuate a ratchet mechanism inside the nail (Albizzia nail; DePuy, Villeur- banne, France). Cole et al 6 used the concept of rotation through the callus but modified the internal mechanism to produce the Intramed- ullary Skeletal Kinetic Distractor (ISKD) (Orthofix, McKinney, Texas). Schiedel et al 10 found that 52 of 58 femoral lengthening proce- dures with the ISKD were successful, and Simpson et al 11 achieved the desired femoral lengthening in 32 of 33 limbs. In a previous study conducted at our centre, we reported a mechanical failure rate of 6.2% for the ISKD (15 of 242 ISKDs). 12 Baumgart et al 5 devel- oped the first practical motorised lengthening nail to become commercially available (Fit- bone; Wittenstein Intens, Igersheim, Ger- many). In 2011, the Precice nail (Ellipse Technologies Inc., Irvine, California) was approved for use in the United States. This nail has an internal lengthening mechanism that is activated by an external magnet. When an external fixator is used to lengthen the femur, the device can be aligned with the mechanical or the anatomical axis. Aligning it with the anatomical axis theoretically leads to medialisation of the knee joint (lateralisation of the mechanical axis line) 1 (Fig. 1). There- fore, most femoral external fixators are aligned with the mechanical axis. When lengthening intramedullary nails were introduced, their lengthening, by necessity, occurred along the anatomical axis. However, in a study of 32 femora that underwent lengthening over a nail, no consistent mechanical axis deviation (MAD) was observed. 4 Most of the nails used were small in diameter and relatively flexible. As a result, varus alignment of the proximal femur often occurred during lengthening, and this might have countered any predicted lateral mechanical axis deviation. Totally implantable intramedullary lengthening devices also lengthen along the anatomical axis, but as they are larger in diameter and stiffer than standard intramedullary nails, they do not allow as much tilting of the proximal femur into varus. We evaluated the change in mechanical axis deviation that occurs after isolated femoral lengthening with the ISKD. Based on the