1.1 Introduction

The roots of scoliosis recognition and treatment date back to antiquity. The oldest known documentation of spinal deformity correction was written between 3500 BCE and 1800 BCE in an ancient mythological Hindu epic called Srimad Bhagwat Mahapuranam. In this epic, Lord Krishna applies axial traction to correct a hunchback in one of his devotees, Kubja, by pressing her feet down and pulling her chin upward. The original Sanskrit translated to English reads:

“To shower the fruits of his blessings, Happy Lord Krishna decided to straighten Kubja, Who was deformed from three places. He pressed her feet by his foot, Held her chin by two fingers and pulled her up. By the touch and pull of Lord She became a beautiful straight woman.” ¹

Kubja’s spine was described as deformed in “three places,” likely indicating a form of kyphosis or kyphoscoliosis, treated by what may have been the first known use of spinal traction for deformity correction.

In the 5th to 4th century BCE, Hippocrates, the father of modern scientific medicine, was one of the first pioneers to thoroughly study the anatomy of the human spine as well as delve into specific pathologies including tuberculous spondylitis, posttraumatic kyphosis, scoliosis, vertebral dislocation, burst fractures, and fractures of spinous processes. ² In his book On the Nature of Bones, he describes in detail the components of the spine including intervertebral discs, ligaments, and muscles, and remarks the role of the spine in maintaining the erect posture of man. ³ Hippocrates was also the first to divide the spine into cervical, thoracic, and lumbar sections and wrote about the natural lordosis of the cervical and lumbar spine and the kyphotic nature of the thoracic spine. ^{4 , 5} Based on logical reasoning and close observation of athletes and cadavers on battlefields, he suggested possible ideas for the treatment of spinal deformities, including the Hippocratic ladder and board, both of which used traction and pressure to reduce spinal curvatures.

Centuries later in Ancient Greece, another distinguished physician, Galen (200-130 BCE), added to the works of Hippocrates through his own studies of the spine and its curvatures, which led to an accurate model of the vertebral column and spinal cord. ⁶ He contributed significantly to the knowledge of spinal disease such as tuberculosis and injuries of the spinal cord, and his findings remained among the most thorough in the field for centuries until much more recent advances in technology were made.

One early example of nontuberculous scoliosis dates back to Medieval England. ⁷ In 2012, archaeologists discovered the remains of King Richard III (1483–1485), whom Shakespeare had famously described as “hunchbacked” in his plays. On the contrary, computed tomographic reconstruction of Richard’s spine revealed a large right-sided scoliotic curve, which likely would have progressed throughout his lifetime. Further studies of his spine revealed that the curve probably first developed in early adolescence, confirming one of the earliest known cases of adolescent idiopathic scoliosis.

1.2 Lewis Sayre, the Father of Orthopaedics

The idea of using traction and suspension for spine correction continued in the 1800s. Orthopaedists in the days of the Civil War were considered primarily cast and brace makers, providing stability to fractures through indirect reduction and external stabilization techniques. Graduating with a medical degree from the College of Physicians and Surgeons in New York in 1842, Lewis Albert Sayre became one of the most prominent orthopaedic surgeons in the United States. Sayre established the first academic department of orthopaedics at Bellevue Medical College and served as its first professor of orthopaedics in 1853. ⁸ Later in 1898, Bellevue Medical College merged with the New York University School of Medicine.

During his career, Sayre became well known for his successful resection of tuberculous arthritis of the hip and the invention of body casts, coined “Sayre jackets,” for the treatment of tuberculous spondylitis (Pott disease) and scoliosis. ⁸ For the latter, patients were partially suspended by their shoulders and head from a hoist connected to the ceiling, lifted into the air to straighten the spine as far as pain would allow, then wrapped in plaster of Paris bandages that formed a rigid cast. ⁹ The Sayre jackets paved the way for modern body casting and bracing for scoliosis.

In 1877, Sayre developed a mechanical model of scoliosis that demonstrated his three-dimensional (3D) understanding of scoliosis. The model consisted of a frame with several horizontal elastic bands holding up a makeshift spine by its spinous processes, and pressing on a brass knob on top of the frame would exert horizontal forces through the bands, creating a torsional result and spinal curves closely resembling those seen in adolescent idiopathic scoliosis.

1.3 Operative Treatment

Despite the widespread use of external stabilization techniques, only a limited effect was seen in scoliosis correction. More severe and sometimes life-threatening cases of spinal deformity required more invasive means of treatment. At the turn of the century, early luminaries attempted to stabilize the spine internally. While early surgeries were for trauma, they paved the way for deformity treatment.

Among the first was Berthold Ernest Hadra (1842–1903), a German surgeon who served in the Prussian army before immigrating to the United States to settle in Texas. ¹⁰ In 1891, Hadra demonstrated the use of interspinous wiring for a case of an old unstable C6–C7 cervical fracture, where he reduced the dislocation and then interlinked the spinous processes with a silver wire. ¹⁰ Although his first patient did not have tuberculosis, he believed that the same treatment could be used for patients with Pott disease and published a paper advocating its use in such cases, which he presented before the American Orthopaedic Association in September 1891. ¹⁰

Two years later, the first internal fixation for Pott disease was performed in Paris by Antoine Chipault. He followed a similar method of using silver wiring through holes in the spinous processes for stabilization after reducing the deformity with traction and direct pressure and by 1896 had performed this procedure on five patients with Pott disease. ¹⁰

1.4 Russell A. Hibbs and Frederick H. Albee

The idea of stabilizing the spine using a bony fusion was first proposed at the start of the 20th century by two surgeons working in New York City. Russell A. Hibbs was born in Kentucky, graduated from the University of Louisville School of Medicine, and then moved to New York City in 1893. He was appointed superintendent and house surgeon at the New York Orthopaedic Hospital in 1894 and became its surgeon-in-chief in 1900. ¹¹ During his career, Hibbs served as a major in the medical corps in the First World War, traveled weekly to give lectures to young surgeons in training at Walter Reed Hospital and the Columbia University College of Physicians and Surgeons, authored over 200 original research articles including 25 on spinal deformity treatment, and examined Franklin D. Roosevelt, pronouncing him fit to be president. ¹¹

During Hibbs’ time, tuberculosis was the leading cause of death in the western world, and he devoted much of his time tending to patients suffering from Pott disease. ¹² After successfully fusing a knee in a patient with poliomyelitis by mortising the patella, he set out to apply the idea to the spine. ¹¹ Working with George Huntington from the College of Physicians and Surgeons, Hibbs developed a method of spinal arthrodesis where he mobilized and longitudinally transposed spinous processes to the interspinous gap to create a continuous bony fusion. He believed the “bony bridge” would prevent kyphosis and provide cantilever support to the deformed spine. ¹³ He first performed the procedure in 1911 on a 9-year-old boy with lumbar Pott disease, whose 3-month postoperative radiographs showed successful bony fusion. ¹³ He further applied his method to idiopathic scoliosis in 1914, reasoning that a progressive curvature could be arrested if the vertebrae were fused to each other. ¹⁴

At the same time and in the same city as Hibbs, Frederick H. Albee developed a similar method for spine fusion. Albee was born in Maine and grew up learning the principles of grafting from fruit trees on his family farm. After graduating from Harvard Medical School in 1903, he became the chief surgeon of the New York Postgraduate Medical School Clinic. He brought the concept of grafting to orthopaedic surgery. In 1906, he performed a successful bone grafting operation on a hip of a patient suffering from rheumatism. In 1911, simultaneous to Hibbs, he developed the Albee technique, performing a successful spinal fusion using the patient’s autologous tibial graft as a bridge after splitting the spinous processes. ¹⁵ The following year, he invented the Albee bone mill, a tool that significantly decreased the time needed to obtain a bone graft. ¹⁶ His bone mill and attachments evolved into the modern power equipment now commonly used in operating rooms around the world.

Albee was well known in his field and was sought after by many leading institutions. He demonstrated his methods in various military hospitals in France as well as in the Royal Medical Society of London. ¹⁷ He set up the U.S. General Hospital Number 4 in Colonia, New Jersey, in 1918 where he served as the chief surgeon. During his career, which coincided with World War I, he treated thousands of orthopaedic injuries, implemented a rehabilitation program for wounded soldiers, and created the New Jersey Commission for Rehabilitation, where he served as chairman for 23 years. ¹⁷ Throughout his life, Albee exemplified his motto: “Never train around a disability that can be removed.” ¹⁸

1.5 John Robert Cobb and the Cobb Angle for Scoliosis

Not long after Hibbs’ and Albee’s development of fusion techniques, John Robert Cobb became prominent in the field of scoliosis treatment. After graduating from Brown University with a degree in English literature, Cobb decided to pursue a career in medicine and received his MD from Yale University, after which he went on to complete his residency in orthopaedic surgery, drawn to the field by his strong interest in mechanics. ¹⁹

In 1934, Cobb was appointed Gibney Orthopaedic Fellow at the Hospital for the Ruptured and Crippled in New York. Orthopaedics in its early years was mostly a nonoperative specialty. The “Ruptured” in “Ruptured and Crippled” represented the vast numbers of hernia surgeries performed at the hospital. In 1949, Philip Wilson Sr. merged the orthopaedic aspects of the hospital in Cornell Medical School, resulting in the Hospital for Special Surgery. During his career, Cobb headed the Margaret Caspary Scoliosis Clinic but is probably most remembered for the Cobb periosteal elevator. ²⁰ Cobb concluded that the best method for scoliosis treatment was with the use of turnbuckle plaster jackets in addition to spine fusion, which at the time were often performed with the child in a cast during the operation. Cobb also noted that not all children with scoliosis displayed curve progression, which led him to advocate for a period of observation before proceeding with surgical intervention. ²⁰

In 1948, Cobb developed a method of measuring the angle of curves in scoliosis, the “Cobb angle,” which is still widely used. In the coronal view, every scoliotic curve has an apical vertebra (most displaced from midline with the least tilted end plates) and superior and inferior end vertebrae, which have the most tilted endplates. Cobb measured the angle of curvature by drawing lines parallel to the upper border of the superior vertebral body and the lower border of the inferior vertebral body, then drawing perpendiculars from these lines to cross each other; the angle formed by the crossing lines was the Cobb angle. ^{20 , 21} In an S-shaped curve, the lower end vertebra of the proximal curve would serve as the upper end vertebra of the distal curve. To be diagnosed with scoliosis, a patient must have a curvature with a Cobb angle of greater than 10 degrees.

1.6 Fusion, Casting, and Prolonged Bedrest

Similar to Dr. Cobb’s use of turnbuckle plaster jackets after fusion surgery, the idea of bracing, casting, and general immobilization techniques postoperatively was widely accepted in the mid-20th century. In 1946, Walter Blount and Albert Schmidt developed the Milwaukee brace, a full-torso cervicothoracolumbosacral orthosis (CTLSO). ²² Though historically used to immobilize patients after surgery, it is used today to prevent progression of scoliosis in children who have not reached their growth spurts and are required to wear the brace 23 hours per day. ²² The cervical component of these braces in growing children had an unfortunate side effect of deforming and impairing proper mandibular development.

In the late 1950s, Joseph C. Risser developed two different postoperative casts. His turnbuckle cast was a lighter, more contoured version of previous turnbuckle casts, which were cumbersome and heavy. ²³ He later developed the Risser localizer cast, which consisted of a rigid three-point mold and a pusher that applied posterolateral pressure to the rib angulation, intended for use immediately after surgery. ²⁴ The cast allowed patients to be ambulatory during the prolonged immobilization period. Risser is also known for his development of the Risser scale, which assesses spinal maturity by means of evaluating the ossification of the iliac apophysis, which coincides with that of the vertebral plates. ²⁵

1.7 Paul R. Harrington

Despite the development of novel fusion and external bracing techniques in the mid-20th century, successful treatment was limited by long periods of immobilization and frequent reports of infection, fusion failure, and loss of correction. ²⁶ Soon, the first implantable instrumentation allowing for stabilization of the deformed spine was introduced by Paul R. Harrington of Houston, Texas.

Dr. Harrington was born in Kansas and was known in his youth to be an outstanding athlete, serving as the captain of the University of Kansas basketball team and winning the regional championship in the javelin throw. ²⁷ After receiving his MD degree from the University of Kansas Medical School and completing his orthopaedic surgery residency in Kansas City’s St. Luke’s Hospital, he joined the U.S. army, where he served as chief of the orthopaedic service in the 77th Evacuation Hospital during World War II. ²⁸ Later in his career, he also helped found the Scoliosis Research Society and served as its president from 1972 to 1973.

In the post-war years, Harrington worked at Jefferson Davis County Hospital in Houston, Texas, where the majority of his scoliosis patients were victims of the poliomyelitis epidemic. He began to research new treatments to correct scoliosis in polio patients, who were unable to undergo physical therapy due to their illness.

After experimenting with various internal fixation methods, he finally developed the Harrington rod, a stainless-steel rod that would be attached to the spine at each end with hooks and then tightened with ratchets to straighten or distract the spine. The rod offered compression, distraction, and three-point bending forces that were good for both stability and correction. He presented his new implant to the American Academy of Orthopaedic Surgeons in 1958.

In 1963, John Moe presented his validation of the Harrington rod after recognizing that the technique would be successful only if it incorporated a solid bony fusion of the levels under the instrumentation. He used the Harrington rod plus fusion in 66 patients with favorable results, and within the next few years, fusion with instrumentation became regarded as superior to fusion without instrumentation. The Harrington rod grew in popularity and became the standard treatment for scoliosis until the late 1980s. ²⁹

The Harrington rod was not without fault. Early attempts to use the rod without fusing the spine failed due to movement of the unfused spine leading to rod breakage. However, when fusion was combined with instrumentation (even with failure of instrumentation over time), a successful fusion would maintain stabilization. The Harrington rod successfully treated curves without fusion in patients younger than 10 years, and this was the premise for the Food and Drug Administration (FDA) later approving growing rods and MAGEC devices.

Another issue after instrumented fusion for coronal deformity, especially in cases where the rod extended down to the lower spine, was “flatback syndrome.” This occurred where the natural lordosis of the lumbar spine was eliminated, causing pain, sagittal deformity, and difficulty with ambulation. It was not until the mid-1980s when new instrumentation techniques were developed that this event could be avoided.

1.8 Allen F. Dwyer and Anterior Instrumentation

Where Harrington instrumentation and fusion was suited to thoracic and double scoliotic curves, curvature of the lumbar spine was best corrected using the Dwyer method, developed by Allen F. Dwyer of Sydney, Australia. ³⁰ The Dwyer method was the first anterior spinal instrumentation for spine deformity, developed on the principle that scoliosis could be corrected by shortening the convex side of the curve. ³¹

In 1964, Dwyer conducted a two-stage surgery in which he first performed posterior release, resecting the ligaments and capsular structures overlying the facet joints on the concave side, and excising any fibrous and bony ankylosis between the laminae, followed by corrective instrumentation via an anterolateral approach. ³² He used titanium screws that were embedded into vertebral bodies on the convex side of the curve, which were then connected by a titanium cable. The cable would then be put under tension, allowing a stepwise correction of each vertebral body to the adjacent screw, ultimately straightening the convex curve. The intervertebral discs were removed to promote shortening of the convex side of the curve and to aid in spine fusion. Compared to Harrington instrumentation, the Dwyer technique required a shorter fusion length and produced better correction of the lateral curvature and rotation, especially in the thoracolumbar and lumbar spine. ³³

A drawback to the Dwyer technique was its contraindication in patients with a kyphotic spine. The instrumentation had a tendency to shorten the anterior part of the spinal column, thereby causing or worsening kyphosis. Also of note, a later study on 21 children who were followed up for 10 years after anterior instrumentation between 1972 and 1975 showed a frequent rate of pseudarthrosis, associated with failure of the cable and loss of correction. ³⁴ In complex cases, a posterior Harrington rod operation was performed as a planned staged component after the anterior scoliosis surgery.

1.9 Zielke Instrumentation Anterior Surgery

Improving on Dwyer’s anterior instrumentation to obtain better correction of the scoliotic spine, Klaus Zielke introduced his ventral derotation spondylodesis (VDS) instrumentation in 1976, which was aimed mainly at treating progressive, single, major lumbar or thoracolumbar curves in idiopathic scoliosis. ³⁵ Zielke’s method involved placing the screws more posteriorly through the vertebral bodies, which helped both derotate the spine and decrease the incidence of kyphosis.

Later, the VDS method was modified further to develop the Halm–Zielke instrumentation (HZI), which used a threaded VDS rod and a solid fluted rod that allowed for internal derotation and relordosation and improved rotatory stability and postoperative sagittal alignment. ³⁶ Halm–Zielke was a major improvement to the original Zielke VDS method in its ability to eliminate the kyphogenic effect and to provide primary stability.

Around the same time, in Japan, Kiyoshi Kaneda developed a two-rod anterior system that further sought to improve anterior surgery with minimal morbidity.

1.10 Kostuik–Harrington Anterior Instrumentation Utilizing Harrington Screws

In the case of kyphotic deformities, a different anterior instrumentation technique was developed by John P. Kostuik. Dr. Kostuik received his MD from Queen’s University in Kingston, Ontario, and then later served as the Chief of Spine and then Chief of Orthopaedics at Johns Hopkins School of Medicine. He was a past president of the Scoliosis Research Study and founding member and past president of the North American Spine Society.

Kostuik was interested in the biomechanics of spinal deformities and particularly focused on the kyphotic spine. Using a modification of Harrington’s technique, Kostuik relied on distractive forces on the anterior spine in combination with instrumentation. ³⁷ The anterior distraction helped resist compressive loads, and the use of segmental fixation helped minimize sagittal bending.

By 1990, Kostuik was able to report successful use of the technique in a wide variety of cases including burst fractures, posttraumatic kyphosis, Scheuermann disease, rigid round backs, congenital kyphosis, flatback syndrome, postlaminectomy kyphosis, kyphosis secondary to tumor, and osteoporosis-related kyphosis. ³⁶ Of the 279 cases reported, complications included 35 screw breaks and two fractures of distraction rods. ³⁶ In a publication in the Iowa Orthopaedic Journal, he described and illustrated in detail the specific surgical technique he used to treat each case. Kostuik’s method resulted in minimal morbidity, and “the wide range of application, ease of adaptability, and versatility” ³⁶ of his technique on kyphotic deformities of the spine were major steps for anterior spinal instrumentation.

1.11 Luque Segmental Fixation

In 1976, Eduardo Luque from Mexico City introduced a new, innovative concept in surgical scoliosis treatment: segmental spinal instrumentation. Luque saw spinal deformity as a multiplanar issue and developed a method that involved multiple points of fixation, which also had the advantage of stress load distribution. From a posterior approach, he used sublaminar wire loops to secure prebent stainless steel rods in place. ³⁸ Increasing the points of fixation in most scoliosis cases provided adequate and stable correction, and by doing so Luque hoped to reduce the need for external immobilization. This was especially important for patients in Mexico, where the hot climate would make brace compliance particularly difficult.

The Luque posterior technique was also used for severe kyphosis, as seen in three deformity cases of adolescents with myelomeningoceles that caused a thoracolumbar kyphosis between 90 and 120 degrees and a compensatory thoracic lordosis. ³⁹ These were corrected with wedge osteotomies of the gibbus deformity, segmental sublaminar wire fixation of the Luque rods, and spondylodesis with bone graft, all from the posterior aspect without requiring an anterior release. The spinal cord and dura remained intact in each case, and at 30-month postoperative follow-up, there was no notable progression of the curves. For further stabilization of the instrumentation, L-shaped rods were later devised to prevent rod migration.

Occasionally, the Luque method of sublaminar wiring would be used with Harrington rods, a technique sometimes referred to as “Tex-Mex” as a tribute to Harrington’s Texan background and Luque’s Mexican heritage. This method was popular, as both axial and transverse loading were used to correct the scoliosis, resulting in a very strong construct. ⁴⁰

Fixation of the lumbosacral spine and pelvis in the setting of pelvic obliquity was a particular challenge for which both Harrington’s and Luque’s methods were not quite suited. In 1976, Ben Allen and Ron Ferguson developed the Galveston technique, which involved inserting a long, contoured rod through the posteroinferior iliac spine into each ilium between the inner and outer tables, extending through the ilium to the area above the sciatic notch. ⁴¹

An alternate technique for segmental fixation was later developed by Denis Drummond and his colleagues at the University of Wisconsin, which they named the “Wisconsin system.” ⁴² The technique consisted of passing doubled closed-loop wires through holes drilled at the base of the spinous processes. The wires had steel buttons that were pulled against the spinous processes, creating a pullout strength just as strong as Luque’s sublaminar wires. ⁴¹ Contoured rods would then be segmentally fixed to the spine, preserving sagittal curves. The Wisconsin system was easier to implant compared to Luque’s sublaminar wires and also had the advantage of being neurologically safer. ⁴¹

1.12 Cotrel–Dubousset Instrumentation and 3D Concepts of AIS

The next advancement in segmental scoliosis correction came in 1983, when Yves Cotrel and Jean Dubousset introduced a new method of posterior instrumentation that would provide strong fixation and adequate reduction of the scoliotic curve, as well as derotation, while minimizing cord damage by avoiding the sublaminar space.

The Cotrel–Dubousset technique consisted of placing hooks on the lamina or pedicles of the spine and then independently inserting two parallel cylindrical rods in the convexity and concavity of the curve which would attach to the hooks and be locked by blockers, allowing for progressive straightening of the curve. ⁴² The use of multiple hooks allowed the application of compression and distraction over different areas within the same rod and applied the principles of segmental fixation without the need for sublaminar wires. The rods could also be cross-linked by transverse rods creating a quadrilateral frame, which added further stability to the construct and allowed for correction of rotation, a powerful new step in the correction of scoliosis.

Cotrel and Dubousset correctly saw the scoliotic spine as a 3D structure and created their system to help improve the alignment of the spine in the coronal, sagittal, and axial planes. Untwisting the vertebrae allowed for natural reduction of the rib prominence, as the ribs associated with the deformity were attached to the vertebrae being derotated. ⁴³ External immobilization (cast or brace) was never required.

Cotrel and Dubousset’s method was later modified into several other similar devices that incorporated two rods and transverse connectors and multiple points of fixation, including Harry Shufflebarger’s and Jurgen Harms’ MOSS-Miami system, Asher’s Isola hybrid construct system, and the Texas Scottish Rite system. ⁴⁴

1.13 History of Pedicle Screws and Plates: King, Boucher, Roy-Camille

Spinal fixation by hook instrumentation was soon followed by the use of pedicle screw fixation, a technique that offers a secure grip on all three columns of the vertebrae. The use of screws in spine surgery began with many failures, but over time, modification and improvement of screws, rods, and plates led to a relatively safe and effective method for spinal stabilization in scoliosis, spondylolisthesis, pseudarthrosis, unstable fractures, postdecompressive instability, and tumor-associated instability of the spine.

The earliest use of bone screws for internal spinal fixation dates back to Jame Tourney in 1943 and Don King in 1944. King developed facet screws, 1 inch long for men and three-quarters of an inch for women, and described their placement as “parallel to the inferior border of the lamina and perpendicular to the facet joint.” ⁴⁵ However, facet screws had a very high pseudarthrosis rate, reported between 10% in L5–S1 placement and 55% in L4–L5 placement, and this discouraged many surgeons from using screw fixation.

In 1959, Harold Boucher was the first to describe placement of screws in pedicles. He developed longer (1.5–2 inches) stainless steel screws that were placed through the inferior facet into the pedicle and vertebral body below, significantly improving fixation without a single instrumentation failure in 160 cases of single-level fusion. ⁴⁵

In 1970, Raymond Roy-Camille of France introduced the use of pedicle screws connected to a posterior plate. Following anatomic studies that showed the interpedicular distance to be approximately 2.6 cm, the plates were designed with collar reinforced holes spaced 1.3 cm apart which allowed placement of 4.5-mm screws in the pedicles, as well as additional smaller facet screws if needed. ⁴⁵ The cobalt chrome or stainless steel plates were specially designed for specific purposes, including one for preventing slips in high-grade spondylolisthesis and another for lumbosacral fusions, the latter of which had a reported 100% success rate. ⁴⁵ In the United States, Harrington used a modified version of his original approach using transpedicular screws to correct various cases of spondylolisthesis (including rapidly progressive spondylolisthesis associated with myelomeningocele) and to manage unstable lumbar fractures. Pedicle screws continued to grow in popularity and became a major part of lumbar and lumbosacral fusion procedures.

Later, in 1983, Arthur D. Steffee developed a modified version of Roy-Camille’s plate that allowed plates to be used anywhere between the lower thoracic spine and sacrum. Steffee plates were contoured to the spine and varied in length, with each plate consisting of a long slot with up to five “nested holes” in the slot where the screws could be placed. This became known as the variable screw placement system. In 1988, Luque further modified the plate and screw, introducing a semirigid cannulated screw and slotted plate system for interpedicular fixation, which allowed for up to 15 degrees of angulation at the screw–plate junction. ⁴⁶