The athletically active adolescent places high demands on his or her lumbosacral spine, which serves as the source of dynamic power during a golf or baseball swing, a boxer's punch, a gymnast's landing, or a weight lifter's power-clean . During overhead motions such as a tennis serve or pitching, the spine acts as key component of the kinetic chain transferring force from the lower limbs through the upper limb [2•]. Pain and dysfunction of the lumbosacral spine typically represent clinical manifestation of microtraumatic mechanical injury. However, in the young athlete such symptoms may represent signs of atraumatic causes of lumbar pain such as tumor , infection , or rheumatologic disorders [5••]. Mechanical lumbar pain occurrence ranges from 1% to 30% in adults [6–8], whereas back injuries have been estimated to occur in 10% to 15% of athletic adolescents [9,10]. Prevalence varies among sports participation and further varies depending on the position played. For example, the incidence of lumbar injury in gymnasts is suggested to be 11%, but soars as high as 50% in football linemen . An accurate diagnosis is not always established when pursuing mechanical lumbar pain, thus leading to dissatisfying treatment effects .
The lumbar functional spinal unit is composed of two vertebral bodies and intervening intervertebral disc, and the associated posterior elements. A variety of confluent intraspinal ligaments envelope the functional spinal unit to assist in stabilization. Collectively, this unit acts to transmit axial loads from one vertebra to the next, afford flexion-extension movements, provide stability, and prevent translatory and torsional shear . The adult lumbar functional spinal unit can withstand axial compressive loads between 3000  and 10000 Newtons (N)  before suffering vertebral endplate fracture. However, endplate failure can occur due to fatigue at loads between 30% and 80% of ultimate failure strength with repetitive movements of 100  to 2000  repetitions. Both the intervertebral disc and posterior elements can be injured during forced axial rotation as the axis of rotation moves from the intervertebral disc posteriorly to the impacted zygopapophyseal joint (z-joint) . At this moment the disc is subjected to a combined torsional and lateral shear force resulting in an annular tear . Risk of injury is heightened during rotation combined with lumbar flexion . The posterior z-joints can suffer capsular strain, tear, avulsion, or avulsion-fracture .
The adolescent spine contains areas of growth cartilage and immature ossification centers susceptible to compression, distraction, and torsion injury [5••], and during growth these areas are often the weakest link of force transfer . As adolescents experience their growth spurt, imbalances develop across the functional spinal unit due to the inability of the musculotendinous soft tissues to keep up with the growth of the bony elements of the lumbosacral spine. Consequently, injurious forces are imposed on susceptible growth tissues . Iliopsoas inflexibility, tight thoracolumbar fascia, genu- recurvatum, thoracic kyphosis, femoral anteversion, and deconditioned abdominal musculature increase lumbar lordosis leading to increased shear force across the disc [5••]. Poor coordination of lumbar core stabilizing musculature decreases the spinal unit's resilience to injury. The frequency and variety of athletic activities engaged by the adolescent athlete results in repetitive microtrauma to the components of the athlete's spine. A golf swing, for example, involving primarily torsional movement produces over 6000 N compressive force across the L3-4 segment . Similar compressive forces are generated at L3-4 spinal segment during rowing , cricket , and blocking maneuvers performed by football linemen . However, shear forces in the sagittal plane disrupt annular fibers leading to structural discogenic injury at much lower loads .
During athletic activities, the lumbar spine is exposed to potentially injurious forces especially when repetitive demands are placed on the constituents of the spine. Repetitive movements in the sagittal plane precipitate injury of the posterior elements . Flexion-extension movements can lead to abnormal stresses placed on the pars interarticularis [28,29], lamina , spinous processes , interspinous ligament , and ring apophysis [33,34]. Ossification progresses posteriorly and the superior portion of the pars interarticularis may be congenitally incomplete especially at L5 during the teenage years . A lytic injury ensues due to repetitive microtrauma from the abutting inferior articular process of L4 .
The structural and biomechanical differences between the adolescent and adult lumbosacral spines are reflected in the epidemiology of adult and adolescent lumbar spine injuries. Micheli and Wood  retrospectively found that 47% of young athletes with persistent lumbar pain were suffering from injury of the posterior elements such as spondylolysis or spondylolisthesis in contrast to approximately 5% of the adult population with low back pain. Only a minority of the young athletes presenting with lumbar pain were diagnosed with a symptomatic intervertebral disc injury. In contrast, this was the most common etiology for persistent lumbar pain in the adult patients . Interestingly, 26% of the adolescent cases of Micheli and Wood  had been diagnosed with hyperlordotic mechanical lumbar pain without structural evidence of osseous posterior element injury. Baastrup's disease or abutment of lumbar spinous processes, lumbar interspinous ligamentous injury [37–39], and injury of the vertebral body [40••] have been reported in this age group, and may have represented a portion of the 26% of hyperlordotic mechanical lumbar pain subjects in the study by Micheli and Wood .
The spine specialist responsible for diagnosing and treating the teenage athlete must have a firm knowledge of the biomechanics, pathophysiology, and diagnostic and therapeutic algorithms for adolescent spine injury. This short review from the perspective of an interventional spine physiatrist covers the differential diagnoses for youngsters presenting with persistent lumbar pain upon sports participation. The primary focus of this article is the discussion of the nonspondylolytic diagnoses that must be maintained in the practitioner's lexicon in approaching this specific patient-athlete. Discussion of tumor and infection are excluded to focus on the above topic. The following sections are arranged in descending order of conditions that will likely clinically mimic pars injury.
Baastrup's Disease and Interspinous Ligamentous Injury
Baastrup's disease or “kissing spines” is due to chronic contact between adjacent spinous processes leading to sclerosis and flattening of the opposing surfaces . The intervening interspinous ligament contains a thin central portion that undergoes fatty degeneration with aging . During spinal flexion the interspinous ligaments are maximally stretched. Subsequent extension allows adjacent spinous processes to approximate while exposing the interspinous ligament to compressive forces . Repetitive lumbosacral flexion and extension movements strain the attachments of the interspinous ligaments impinging these inflamed ligamentous insertion sites . These areas of soft tissue strain develop vascularized granulation tissue that contain an abundant nerve supply .
The incidence of Baastrup's disease and inter-spinous soft tissue injury in collegiate athletes has been calculated to be 6.3% , and most commonly affects gymnasts . Athletes with this injury will present with midline lumbar pain that is exacerbated by extension and decreases with flexion [37,39]. However, extreme lumbar flexion can stretch painfully inflamed interspinous fibers eliciting lumbar pain . Point tenderness exists over the involved interspinous space and sometimes the adjacent spinous process [37,39]. Although any lumbar level can be affected [37,39,41], the L4-5 segment is the most common .
The diagnosis is based primarily on physical examination findings and historic features. Plain radiography may reveal sclerosis, flattening, and reactive eburnation of the opposing spinous process' edges . Such bony change may not be present in the adolescent spine due to adequate disc height and lack of osseous contact between the spinous processes. Soft tissue structural change of the interspinous ligament can be appreciated on MRI on sagittal T2-weighted sequences . Subsequent positron emission tomography may demonstrate increased fluorine 18-deoxyglucose uptake in the interspinous ligament-spinous process interface consistent with increased metabolic activity . Fluoroscopically guided diagnostic extraspinal injection of local anesthetic can confirm the interspinous ligament injury as the source the athlete's symptoms .
The treatment algorithm we employ involves a tiered-approach. The initial therapeutic tier involves physical therapy for lumbar stabilization, core conditioning, and flexibility training, and avoidance of exacerbating activities and initiation of nonsteroidal anti-inflammatory drugs (NSAIDs). If symptoms persist, the diagnosis is first confirmed utilizing a diagnostic injection and then followed by one or two therapeutic injections of 1 mL of betamethasone combined with 0.25 to 0.5 mL of 4% xylocaine separated by 2 to 3 weeks . Surgical excision of one or both offending spinous processes is reserved for the young athlete with persistent symptoms who wishes to continue athletic participation . These surgical outcomes may not mirror those seen in adult patients .
Lumbar Spine Osseous Abnormalities
Although injury of the lumbar pars interarticularis is the most common etiology of persistent adolescent lumbar pain , additional osseous injuries occur in 3% of affected athletes, and 14% have injury of another structure rather than the pars [40••]. The other affected areas include the vertebral body, sacral ala, spinous process, sacroiliac joint [40••], and lumbar facet joint [44,45]. The injury mechanism involves repetitive microtrauma of skeletally immature structures without sufficient rest leading to premature return to activity culminating in overt fracture of the subchondral bone . The above-mentioned percentages, however, may underestimate the true occurrence as these calculations were made from cases in which nuclear imaging detected an abnormality supportive of lumbar pain. No further diagnostic intervention such as physical examination findings or precision spinal injection results were reported [40••].
Increased uptake on single-photon emission CT (SPECT) imaging over the sacral ala may indicate the presence of an articulating joint between the sacral ala and L5 transverse process in cases of a hemisacralized L5 vertebral body [40••]. Injection of this articulation under fluoroscopic control may confirm the source of symptoms and provide short-term benefit to allow participation in physical therapy. Ring apophyseal injury as documented by CT can be detected by nuclear imaging studies due to increased radiotracer uptake [40••]. A transforaminal epidural injection of both corticosteroid and local anesthetic may provide lasting benefit, again allowing active participation in a core stabilization program. Small avulsion fractures of the tip of the spinous process confirmed by CT can be a source of lumbar pain, especially in gymnasts [40••]. An avulsion injury such as this may represent one example of the spectrum of injury to this area from interspinous ligament sprain, to spinous process sclerosis, to avulsion, and finally eburnation due to bony abutment. Sacroiliac joint pain occurs less frequently, and isolated reports of sacral fracture [40••] and articular process fractures have been published [44,45].
An intervertebral disc can become painful due to internal derangement or frank herniation. Stress within the annulus can result in circumferential tears which can coalesce to radial tears. Chemically sensitized nerve endings located in the outer annular fibers can relay biomechanical and biochemical triggers as nociceptive input stemming from the disc . Further strain incurred by the disc can result in herniation of the nuclear tear through a radial tear triggering an inflammatory response irritating the nearby dura, posterior longitudinal ligament, and nerve roots . In the skeletally immature athlete, the disc may herniate vertically through the vertebral endplate or ring apophysis due to an axial load performed with concurrent forward flexion [21,47].
Approximately 10% of persistent lumbar pain in the young athlete is discogenic in origin [10,37,47,48]. The L4-5 and L5-S1 segments are most commonly affected [47,48] and endplate herniations occur more commonly at the thoracolumbar junction . Weight lifting, wrestling, gymnastics, and collision sports are risk factors for disc injury [21,47,48]. The athlete typically presents with acute onset of lumbar pain that is midline or across the lumbar region. Nerve root involvement is rare as most adolescent disc herniations are central rather than posterolateral . Pain is exacerbated by prolonged sitting, driving, coughing, and sneezing, and may be alleviated mildly by change of position or lying in the lateral decubitus position. Provocative maneuvers such as pelvic rock or sustained hip flexion  can elicit symptoms. Straight leg raising can result in lumbar pain perhaps due to dural or posterior longitudinal ligament irritation in the presence of a central focal herniation .
Although no physical examination maneuver is pathognomonic for discogenic lumbar pain , history and physical examination must help the clinician assign meaning to imaging findings. MRI will detect morphologic changes of the intervertebral disc and allow differentiation among protrusion, extrusion, and sequestration of disc material. Yet, 20% of asymptomatic individuals in this age group will manifest disc abnormalities on MRI . If the athlete sustains an injury mechanism straining the disc coupled with acute or explosive onset, and pain that behaves most consistently with discogenic lumbar pain, then a disc herniation revealed by MRI is probably responsible for the symptomatology.
The initial treatment tier is composed of physical therapy for dynamic stabilization and core conditioning to decrease anterior shear and torsional strain across the intervertebral disc. Oral NSAIDs are appropriate to address the inflammatory reaction of disc injury and provide mild analgesia. Stronger analgesics such as mild narcotics or tramadol may be useful for pain control to facilitate sleep. If the athlete does not improve, or if initial symptoms are incapacitating, S1 transforaminal epidural steroid injections may be beneficial. Although conclusive evidence is lacking, it is logical to inject corticosteroid and local anesthetic into the anterior epidural space to bathe the posterolateral margins of the disc where the inflammatory response is ongoing . The greatest success with these injections is achieved when performed soon after symptom onset. Less than 0.5% of adolescent athletes will eventually require surgical discectomy for persistent symptoms .
Apophyseal Ring Fractures
The junction of the vertebral body and ring apophysis is vulnerable to injury in adolescents due to lack of complete osseous fusion. Disc herniation can avulse the fibrocartilaginous ring apophysis from the vertebral body posteriorly into the central spinal canal . Approximately 50% of these injuries are due to acute trauma, whereas repetitive microtrauma accounts for a significant proportion of the remaining cases . Compressive loads can fracture the endplate during flexion resulting in a posterior rim avulsion fracture [5••].
Although intervertebral disc herniation is rare in the adolescent athlete, occurrence with a ring apophyseal fracture is unique to the adolescent athlete [5••,48]. Most patients affected compete in weight lifting or sports requiring repetitive lumbar spine hyperflexion . The inferior apophysis of L4 is most commonly involved, and signs and symptoms are similar to those associated with a central herniated nucleus pulposus . Afflicted young athletes present with lumbar and/or lower limb radicular pain. Avulsion of the cephalic vertebral body is more commonly associated with nerve root injury than avulsion from the caudal portion of the vertebral body. Symptoms are worse with prolonged sitting, coughing, sneezing, or activity . Contralateral straight leg raising is more frequently positive than in patients with a herniated disc .
History and physical examination will likely lead the clinician to suspect acute intervertebral disc herniation. Avulsion of the ring apophysis should be evaluated by computed tomography to better delineate the osseous injury. Magnetic resonance imaging may be inadequate to differentiate between bone and disc material . Plain radiography may reveal a small avulsion fracture from the vertebral body. However, CT will allow assessment of the associated disc via axial views and multiplanar reformatted images.
Initial treatment steps are similar as for disc herniation. Physical therapy is prescribed to address spinal biomechanics, oral NSAIDs for analgesia and inflammation reduction, as well as activity modification and sometimes a soft lumbosacral corset. If symptoms persist or progress, open surgical discectomy and excision of the bone fragment are indicated [5••,48].
Lumbar Scheuermann's Disease
Scheuermann first described the thoracic kyphosis related to three consecutive anterior vertebral bodies wedged at least 5% each, along with associated vertebral end plate changes, Schmorl's nodes, and apophyseal ring fractures [5••]. Lumbar Scheuermann's disease is the atypical lumbar variety due to overuse of the lumbar spine [5••,48]. Repetitive flexion of the lumbar spine results in vertical herniation of disc material, Schmorl's nodes, through the adjacent cartilaginous layer of the ring apophysis .
Athletes participating in sports requiring repetitive flexion and extension such as football, weight lifting, gymnastics, and wrestling are prone to develop lumbar Scheuermann's disease [47,48]. Male athletes are affected more commonly than female athletes , the upper lumbar level of the thoracolumbar junction is the segment most often affected, and usually just one level is involved . Localized lumbar pain is the typical complaint that is exacerbated by forward flexion [47,48]. Physical examination may reveal tight hamstrings  and thoracolumbar fascia , and decreased thoracic kyphosis  and lumbar lordosis .
Lateral radiographic views will reveal wedging of the vertebral bodies and three consecutive levels must demonstrate anterior wedging [47,48]. Other associated findings detectable by plain radiography include endplate irregularities and Schmorl's nodes. A SPECT scan can demonstrate increased osteoblastic activity at one or two levels [40,48].
Treatment is composed of activity modification and a hyperextension lordotic spinal orthotic  to minimize painful symptoms. Oral NSAIDs are useful in conjunction with core stabilization and strengthening and stretching tight thoracolumbar fascia . Successful return to sport is achievable after completion of the above outlined plan of care.
Lumbar Paraspinal Compartment Syndrome
A compartment syndrome of the lumbar paraspinal erector spinae musculature can develop after exertion [53–55]. Cadaveric dissections have demonstrated discrete anatomic lumbar paraspinal fascial compartments . Direct intracompartmental pressures have been measure and found to be elevated in symptomatic individuals . Exertion of the paraspinal musculature leads to increased metabolic by-products and nutrient demand of the exercising muscles. The resulting edema leads to pressure-induced muscle injury spilling creatinine phosphokinase into the bloodstream .
Although the incidence is unknown, lumbar paraspinal compartment syndrome due to athletic activity is categorically rare. Published case reports [53–55] discuss involvement of 20-year-old athletes engaged in recreational exercise such as downhill skiing . Patients complain of unrelenting diffuse lumbar pain. Straight leg raising may exacerbate the lumbar pain, and lumbar the paraspinal muscles may be rigid on palpation [53,55]. Serum creatinine phosphokinase levels are frequently elevated and may reach 30,000 IU . Intracompartmental pressures may double and sometimes reach as high as 80 mm Hg .
Coronal MRI evaluation will reveal signal abnormalities on both T1- and T2-weighted images indicative of intramuscular hemorrhagic necrosis . The treating clinician must have a high index of suspicion for a lumbar compartment syndrome based on onset of symptoms and precipitating activity. Subsequent MRI and laboratory analysis should be employed. Increased intracompartmental pressures can be directly measured confirming the diagnosis.
Adequate intravenous hydration should be instituted to minimize myoglobinuria with concurrent modified bed rest . Once symptoms start to dissipate over a few days, upright activity and ambulation with a soft lumbosacral corset is encouraged . Instrument-assisted soft tissue mobilization can be utilized to restore lumbar fascial extensibility . Fasciotomy may be required in chronic lumbar compartment syndromes. However, acute compartment syndromes can be effectively managed nonsurgically with gradual return to preinjury athletic participation within 14 to 18 weeks .
The spine specialist treating the adolescent athlete must maintain a broad knowledge base of the probable etiologies of persistent athletic adolescent lumbar pain. If imaging studies do not reveal the much anticipated spondylolytic injury as suggested by history and physical examination, one must then consider other etiologies. Skeletal scintigraphy can reveal increased radiotracer uptake in 49% of symptomatic adolescent lumbosacral spines; 65% of these injuries are due solely to a pars injury, 15% are due to injury of an alternate structure, and 4% of another structure in addition to the pars . Subsequent CT can better delineate the structural or morphologic characteristics of the lesion. Diagnostic precision spinal injections may help assign clinical significance to these abnormalities .
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