Drill and screw epiphysiodesis are the two most common techniques for surgical correction of predicted limb length discrepancies 2-5cm. Both procedures require minimal incisions, less than 1cm, (Canale \& Christian, 1990; Metaizeau et al., 1998). Previous studies have demonstrated that both drill and screw epiphysiodesis result in improved outcomes compared to open techniques (Alzahrani, Behairy, Alhossan, Arab, \& Alammari, 2003; Canale \& Christian, 1990). Moreover, alternative approaches such as medial and lateral 8 plates may not sufficiently tether growth, or cause peripheral but not central growth arrest (Stewart et al., 2013). We selected drill vs. screw epiphysiodesis as the two treatment groups as they are both minimally invasive, relatively effective, in common usage, and are thought to differ in costs and other characteristics.

Although outcomes of drill and screw epiphysiodesis exist{{10 Campens,C. 2010; 11 Ghanem,I. 2011}} , the assessments are retrospective non-randomized series, which may be at risk for selection bias, and may not adequately capture all of the outcomes of interest, depending on what data is routinely collected and documented in the medical chart. To our knowledge, no prospective randomized comparison of epiphysiodesis techniques and clinical outcomes has been published accurately assessing how effective each technique is in disabling growth at the physis, or taking into account patient-centered outcomes or cost. Operative measures such as the surgical time and radiation exposure from intra-operative fluoroscopy have not previously been compared. Patient-centered outcomes such as level of pain, activity, and function by measures such PROMIS and Ped-FABS, have not been previously assessed in this population. The proposed research study aims to fill this gap.

This study was designed as a multicenter randomized trial to answer an important clinical question and to do so with a clearly defined objective and validated outcomes. This trial can be executed on a relatively small budget with simple outcome measures, and recruitment of a moderate number of patients at each of a few centers well equipped for research. By involving multiple surgeons and patients from various geographies, we improve the generalizability of this study. Our institution and collaborating institutions have been successful in completing randomized clinical trials in the past. This study will answer a clinical question that is important and current, providing orthopedic surgeons with an evidence-based identification of the ideal technique for treating predicted limb length discrepancies 2-7cm.

This study will also be the first to accurately measure mean growth following both epiphysiodesis techniques, through implantation of tantalum beads in consenting participants. These beads will provide stationary landmarks by which growth can be measured directly, and not inferred. Previous methods of judging epiphysiodesis success have primarily relied on assessing efficacy by estimating growth inhibition through calculation. Tantalum beads have been successfully used in adults and children (Lauge-Pedersen,H. 2006), and although it requires implantation of small radio-opaque beads, it is considered the gold standard when making detailed radiographic measures, and the optimal technique for physeal growth measurements.(Lauge-Pedersen,H. 2006)(Haugan,K. 2012). Over 300 000 beads have been inserted in vivo without significant complications.