Screw-Less U-Rod Pelvic Fixation in Early-Onset Scoliosis

Early-onset scoliosis (EOS) often requires pelvic anchors in growing constructs to control pelvic obliquity and lumbosacral alignment. Achieving stable spinopelvic fixation in very young patients is challenging, as standard iliac screw fixation can be limited by small iliac bone size and risk of screw loosening or prominence. Historically, several screw-less fixation methods were developed to secure the spine to the pelvis in pediatric patients. The Galveston L-rod technique, introduced by Allen and Ferguson in 1984,1 involved bilateral rods inserted into the ilia and was widely used for neuromuscular scoliosis. This method achieved substantial pelvic obliquity correction (often on the order of 50% to 80%) without the use of screws. The Luque “unit rod” (U-rod) introduced in the late 1980s by Bell et al2 built upon the Galveston concept by using a one-piece U-shaped rod spanning both iliac crests, also secured with sublaminar wires and no pelvic screws. Around the same time, Dunn and McCarthy3 described an S-rod technique hooking around the sacral ala (also screw-less) to stabilize the pelvis, primarily for myelodysplasia cases. These early techniques demonstrated that secure pelvic fixation in small children was achievable without screws, but each had limitations (eg, rod migration or difficult rod contouring for Galveston rods, and occasional neurological irritation with the sacral ala S-rod). Modern spinal instrumentation has trended toward pelvic fixation with iliac screws for stronger biomechanical purchase; however, in very young EOS patients, the screw anchors themselves can pose risks (such as loosening, hardware prominence, or injury to small ilia and open triradiate cartilage). Therefore, alternative methods continue to be relevant in the pediatric population. At our institution, a screw-less U-rod iliac straddle construct was developed as an evolution of these historical techniques for EOS cases where standard iliac screws were considered suboptimal. This technique involves bilateral rods that straddle the iliac crests and a transverse connector, forming a rigid U-shaped pelvic anchor without any iliac screw fixation (Fig. 1). The construct has been used at our center in children as young as 2 years old. In this article, we describe the surgical technique and the biomechanical rationale. Local research ethics committee approval was received (REC-598-25).FIGURE 1: Anterior and posterior views of a saw bone pelvis with the U-rod construct in situ.TECHNIQUE Proximal anchors are secured to the ribs or spine. Instead of inserting iliac screws, 2 rods are contoured to cup the outer surface of each iliac crest, 1 rod sitting on the inner pelvic table, and 1 on the outer table. Each rod, typically 5.5 mm diameter titanium, is contoured to straddle each iliac crest and a transverse connector (cross-link) is applied across the 2 rods just above the level of the iliac crest (typically at the L5 to S1 level). This connector locks the rods together, forming a U-shaped frame (the 2 vertical rods on each ilium forming the uprights of the “U” and the transverse rod forming the base). One arm of the U is rotated to clamp the ilium and set screws are used to secure the transverse connector to both rods, thereby fixing the distance between the rods (Figs. 1and 2). With the cross-link in place, the rods are effectively clamped against the outer cortex of the iliac wings. Final tightening of all connectors is performed to ensure the rods are snug against the bone surface of each iliac crest. No screws or hooks are placed into the ilium itself; fixation is achieved by the friction fit and buttressing of the rods against the iliac cortices (Fig. 3). This configuration provides a stable triangular support at the lumbosacral junction: each rod pressing on an iliac crest, and the transverse bar preventing the rods from splaying or moving independently.FIGURE 2: Saw bone demonstration of the U-rod construct.FIGURE 3: Plain radiographs from a patient showing bilateral U-rod constructs for growing rods in a patient with early-onset scoliosis.Indications and Pearls The U-rod iliac straddle technique is indicated for early-onset scoliosis patients who require pelvic stabilization but in whom iliac screw fixation is not ideal due to anatomy or bone quality. This includes many neuromuscular scoliosis cases with pelvic obliquity (eg, myelomeningocele, spinal muscular atrophy) and some idiopathic or syndromic early-onset scoliosis patients with severe curves requiring long fusions. Key technical points include careful rod contouring to achieve an exact fit around the iliac crest (to maximize surface contact and friction) and securing a robust transverse connector under distraction (to press the rods firmly against the ilium). The cross-link should be placed as low as possible near the lumbosacral junction to effectively create the U-shaped clamp. If the rods are properly shaped and the connector tightened, the construct resists intraoperative dislodgment. In very small pelvises, the rods may be downsized (to 4.5 mm) to accommodate the anatomy. Surgeons should also ensure there is no soft-tissue interposition between rod and bone that could reduce fixation strength. Neuromonitoring is used throughout the procedure. Notably, although no screws are placed across the inner table of the ilium, the risk to neural elements or pelvic viscera from this pelvic fixation is minimal. Biomechanical Rationale Extending instrumentation to the pelvis provides a longer lever arm for controlling the spine compared with stopping at L5 or S1. By engaging the iliac crests bilaterally, the U-rod construct gains the mechanical advantage of a wide base, improving the correction and maintenance of pelvic alignment (particularly useful for pelvic obliquity in neuromuscular scoliosis). The U-rod iliac straddle construct creates a rigid triangular frame consisting of the spinal column and the 2 iliac rod struts connected by the transverse bar. The iliac crest is effectively “captured” between the rod and the spine, giving an anterior and posterior purchase on the pelvic bone (similar in concept to how an iliac hook secures the crest from the inner side). This buttresses the lumbosacral junction and helps transmit spinal loads into the pelvis over a broad area. Avoidance of Stress Risers A screw-less rod-on-bone fixation avoids the concentrated stress at bone-screw interfaces, which is a common failure point in pediatric patients with small or osteopenic ilia. Instead, the forces are distributed along the continuous contact between the rods and the iliac cortices. The friction and anatomic conformity of the rod to the bone create a stable interface. By eliminating screws, the U-rod construct removes the risk of screw pull-out or breakage in the immature iliac bone. There is no transgression of the inner cortex of the ilium. U rods are clamped externally rather than relying on a press-fit within a bony channel. Likewise, because the rods do not penetrate the sacroiliac joint, the construct avoids creating stress risers at the junction of implant and bone that might lead to sacral fractures or joint violation. Historical Validation of the Concepts The idea of using contoured rods to achieve pelvic fixation without screws is well established: the Dunn-McCarthy S-rod provided strong lumbosacral leverage by hooking the sacrum anteriorly and posteriorly, and the Luque unit rod provided a U-shaped pelvic support by spanning the ilia. Our construct essentially combines these approaches—extending laterally like a unit rod and clamping the bone like an S-hook. Moreover, modern growing-rod constructs for very young children also use iliac hooks (S-hooks) as distal anchors when iliac screws are impractical. Clinical series have shown that such hook-based pelvic anchors can effectively control pelvic obliquity in EOS, with only a modest incidence of hook migration complications.4 Simplicity and Cost-Effectiveness The construct uses standard rods and a cross-link, without the need for specialized iliac screw implants or instrumentation guides. The technique also allows some flexibility in adjusting the rod position during surgery; as the rods are not fixed until the transverse connector is applied, the surgeon can fine-tune pelvic alignment and rod contour, then lock the construct, which is somewhat forgiving. EXPECTED OUTCOMES Achieving stable fixation to the pelvis allows growing-rod constructs to be used in high-risk early-onset scoliosis patients where the risk of screw cutout would be significant. Bell et al2 showed that the Luque unit rod produced reliable outcomes in neuromuscular scoliosis, with high fusion rates and improved sitting balance. Our screw-less U-rod construct achieves the same final configuration as a unit rod but does so by connecting 2 rods with a transverse link intraoperatively rather than using a pre-welded rod. Thus, the success of unit rod instrumentation in prior series supports the validity of the U-rod approach. The Dunn-McCarthy S-rod technique offers another historical parallel. Instead of going into the ilium, the S-rod hooks under the sacral ala, anchoring the spine to the sacrum without crossing into the iliac bone. McCarthy et al reported on 67 patients using this method, achieving about 53% correction of pelvic obliquity on average and maintaining it at 6-year follow-up, with 0% incidence of lumbosacral nonunion.4 This confirmed that even without iliac purchase, a well-designed rod construct could yield solid fusion and lasting deformity correction in neuromuscular scoliosis. Complication profiles for the S-rod were distinct: a small percentage (3% to 5%) experienced rod migration, and some ambulatory patients (those with intact sensation) developed L5 nerve root irritation due to the rod’s proximity to the foramen. The U-rod iliac straddle technique seeks to avoid those issues by staying lateral on the ilium (away from the spinal canal and nerve roots). In the contemporary era of growth-friendly surgeries for EOS, there has been a resurgence of interest in screw-less pelvic fixation in the form of pelvic hooks. For very young patients who cannot accept large iliac screws, surgeons have used rib-to-pelvis growing-rod constructs with distal S-hooks clasping the iliac crest. A report from a multicenter study confirmed that pelvic hook fixation can effectively control pelvic obliquity during growth, albeit with some expected rate of hook migration or displacement that usually can be managed with minor revisions.5 Our U-rod straddle construct can be viewed as a robust, fusion-oriented version of an iliac hook: essentially 2 large hooks (the rod ends) linked together across the back of the pelvis. The key addition is that by locking the rods together, the U-rod construct likely has superior resistance to forces compared with 2 independent iliac hooks. COMPLICATIONS Despite its advantages, the U-rod iliac straddle construct also has limitations. One consideration is that it relies heavily on precise rod contouring and a tight fit; if the rods are not perfectly moulded to the iliac crest, the fixation strength may be suboptimal. Another limitation is that the construct, by design, occupies the space over the dorsal ilium and can be somewhat prominent. In very thin patients, hardware prominence may lead to skin issues. This risk can be mitigated by trimming excess rod and ensuring adequate soft-tissue closure, but it remains a consideration that screws (placed deeper under muscle) might avoid. It is important that both the anterior and posterior rods are long enough to achieve adequate fixation but not too long as to cause soft-tissue compromise. From a biomechanical standpoint, although the U-rod provides robust multi-planar stability, an argument could be made that dual iliac screws with lumbosacral rod connection (triangular fixation) is still the gold standard in terms of ultimate stiffness. Thus, for a larger, heavier patient (eg, an older adolescent or adult-sized teen with neuromuscular scoliosis), one might favor iliac screws over a rod-on-crest method. Our experience with the U-rod is mainly in smaller, younger patients; its performance in a near-adult frame is untested and it may not provide the same stability. There are commercially available, pre-bent, “S-hooks”, for use in early-onset scoliosis. Our technique has the advantage of being able to utilize remaining lengths of rod, and, may therefore, be more cost-effective than commercially available systems. CASE SERIES At our institution, 22 patients with early-onset scoliosis underwent spinal growing-rod insertion with the U-rod construct between 2017 and 2024. Local ethics committee approval was granted for this study (REC-598-25). The median survival of the U-rod construct was 4 years (range: 0 to 8 y). In 11 of the 22 patients the U rods are still in situ at the time of writing. In 6 patients, the U-rod construct failed. Three of these were due to rod breakage near the cross-link, occurring at 4 to 6 years postoperatively. This was likely due to fatigue failure of the titanium rods. One patient suffered an early pelvic fracture with the U-rod construct pushing through the ilium and underwent revision 2 weeks after insertion. There was one U-rod failure due to infection and one patient had aseptic loosening. Five patients had their instrumentation removed due to other reasons (1 was converted to definitive fusion, 3 had proximal junctional kyphosis, and 1 patient was converted to a different growing-rod construct). This data demonstrate the U-rod construct to be reasonably durable and predictable in this cohort of early-onset scoliosis patients. CONCLUSIONS The screw-less U-rod iliac straddle construct is a viable and effective technique for spinopelvic fixation in early-onset scoliosis when standard iliac screws are impractical or risky, such as in very young or small patients. This method can achieve durable lumbosacral fusion, maintain deformity correction, and minimize complications in a challenging patient population. Biomechanically, the construct capitalizes on proven principles, to provide stability comparable to traditional iliac screw fixation, without the need for hardware insertion into small, immature pelvic bones.