Page 11 - DISCUSSION continued

Device evolution is part of progress. The senior author (DP) had the privilege of being involved in the development of 4 of the implantable lengthening nails that are used today. The senior author was a consultant to the Medinov of the Landinger group (Nancy, France) regarding the Albizzia nail and designed and first implanted their tibial nail (femoral nail developed by Guichet and Grammont). This non-FDA– approved device was used as a compassionate use device in the United States in the mid 1990s by several surgeons. This experience uncovered an essential design problem that led to frequent failure due to wear ratchet gear. Hardening the metal used for this part solved this problem. The current Albizzia has also been strengthened to use cobalt chrome instead of stainless steel to permit greater weight-bearing in bilateral lengthening cases. It is currently marketed as either the Guichet nail or Betzbone device by these 2 surgeons, respectively. Despite the increased strength of cobalt chrome there continue to be fatigue failures of the stainless steel screws due to excessive loading, as a reminder that unprotected weight-bearing until distraction gap consolidation is not a good idea. The senior author was also the first user of the ISKD device after its inventor Dean Cole, MD. As a consultant to Orthofix at that time, the company was advised in the first year of ISKD device use (2001 to 2002), that the lack of rate control was a major problem. Certainly many of the problems of not being able to get the nail going which plagued the Albizzia, were solved by the smaller degree of rotation required to actuate the lengthening. These were replaced by the “runaway” phenomenon of too rapid distraction. Although surgeons worked around this problem by decreasing patient activity, using bulky braces such as hip-knee-ankle-foot orthotics, no fix to the problem was offered by the company. The device was finally withdrawn from the market in 2011. It is unknown whether an ISKD2 with better rate control will be available in the future. The senior author also worked with Arnaud Soubieran while he was developing the Phenix nail. There were many trials and tribulation with the initial mechanism. After Soubieran solved most of these, the senior author introduced this nail to Smith and Nephew and worked briefly as a consultant for them on this device. In 2010, the senior author elected to leave the Smith and Nephew team and to become part of the Precice nail development team headed by Stuart Green, MD.

Between December 1, 2011 and November 1, 2013, 155 Precice nails were inserted into 100 patients at the Paley Institute. In addition to the complications listed in this study of the first 65 Precice nails, there have been no more nail breakages through the welds (partly due to greater vigilance in restricting weight-bearing in bilateral femoral lengthening patients). Therefore, the total number of nail breakages for the first 155 Precice lengthenings is 3. In total, there have been 7 mechanisms of 155 that failed to lengthen, 2 due to operator error by the surgeon’s team in applying the ERC device the wrong way and 5 after meeting excess resistance from the callus. There was also 1 femur fracture that occurred after the study group was closed. The fracture was through a distal AP locking screw at the end of distraction that occurred during physical therapy (the smaller end of the nail bent about 10 degrees at the time of fracture). The distal end of the nail offers 3 locking holes; 2 medial-lateral, and 1 AP. In the femur we intentionally nail short to avoid issues with the femoral bow. This creates a stress riser in the mid femur at the end of the nail. That stress riser is increased by an AP drill hole and screw. Although we only saw this complication in 1 patient, we no longer use the AP locking screw for femoral lengthening. We also frequently avoid this screw in the tibia, as the screw head is so subcutaneous and at risk of being exposed if the wound breaks down. A case in point is the 1 patient in the study series who suffered an accidentally self-induced burn over this locking screw by a heating pad, leading to wound breakdown and a deep infection.

Although the reported study was conducted as a retrospective review, it represents a consecutive series with no cases eliminated. As one of the company consultants, the senior author kept Ellipse Technologies abreast of all problems and complications with the Precice as they occurred. The company acted both responsively and responsibly as the complications of failure to distract in the face of rapid consolidation of callus, and fractures of the nail occurred. Although infrequent in occurrence, this study identified 2 potential failure modes with the first version of the Precice nail (which I will herein call Precice 1 or P1): the junction of the gears to the lead screw, and, the welds of the nail on either side of the drive mechanism. Such device failures were clearly less common than documented failures with the only other FDAapproved cleared device, the ISKD.36 Nevertheless, at the advice of and in consultation with the senior author, Ellipse Technologies immediately began work to design a new nonmodular nail that had a stronger gear to lead screw connection and to eliminate the welds in the outer tube of the nail. The gear-lead screw correction was implemented in May 2013. The 1-piece outer tube with no welds required FDA clearance which was obtained in October 2013. The Precice 2 (P2) was first used in the United States in November 2013 (Fig. 10). The P2 is at least 2 times stronger in bending fatigue strength and has a 3 times stronger coupling between the gears and lead screw. The P2 also has greater distraction capacity (stroke length) and is available in 50- and 80-mm stoke lengths. The P2 is also available in a smaller outer diameter (8.5 mm). To achieve these strength gains, modularity is not a feature of the P2. In the P2 one has to select the type of insertion locking end (piriformis, trochanteric, retrograde, or tibial) and the length of the nail in advance. With the exception of attaching the insertion end to the nail, the procedure for insertion of the nail as well as its indications are the same as for the P1. The greater stroke length, while allowing for greater distraction amount must be guided by the usual parameters that guide responsible continued lengthening: pain, bone formation, joint range of motion and stability, and neurological status.

The future for noninvasively adjusted limb lengthening devices is very exciting. Future innovation will likely produce a bone transport nail to treat bone defects, limb lengthening plate for children with open growth plates, and gradual deformity correction plates. Miniaturization and new mechanisms will allow greater application of such technology. Adjustable nails could eventually replace simple locking nails for trauma, allowing adjustability of length postoperatively.

The same technology as applied to prostheses will also find its way from growing prostheses for bone tumors in children to adjustable length joint replacement for the treatment of arthritis.