A bilateral pars defect, also known as spondylolysis, is a stress fracture affecting the bones of the lower spine. The condition occurs when a small, bony segment of a vertebra fractures on both the left and right sides of the spinal column. This defect is a common source of lower back pain, especially in younger individuals and adolescents who participate in sports that place high demands on the spine.
Understanding the Anatomy of the Pars Defect
The spine is composed of stacked bones called vertebrae, each featuring various bony projections and structures that allow for movement and stability. The pars interarticularis, or simply the pars, is a narrow bridge of bone located between the superior and inferior articular processes of a single vertebra. This small segment connects the facet joints, which link one vertebra to the next.
A pars defect is a stress fracture in this bony bridge, compromising the structural integrity of the vertebra. The condition is most commonly found in the fifth lumbar vertebra (L5), which sits at the base of the spine just above the sacrum. When the fracture occurs on both the left and right sides of the same vertebra, it is classified as a bilateral pars defect. This double-sided break weakens the vertebra’s ability to maintain its position.
The term for a defect in the pars interarticularis is spondylolysis. When viewed on an oblique X-ray, the pars forms the neck of an imaginary “Scottie dog,” and the defect appears as a collar. The small size and isolated location of the pars make it vulnerable to fatigue fractures from repeated mechanical loading.
Common Causes and High-Risk Activities
The development of a pars defect is typically not the result of a single traumatic event but rather a gradual process of microtrauma and overuse. Repetitive movements involving hyperextension and rotation of the lower back create cyclical stress that exceeds the bone’s capacity to repair itself. Over time, this constant loading leads to a fatigue or stress fracture in the pars interarticularis.
Adolescent athletes are disproportionately affected because their developing spines are more susceptible to injury during growth spurts. Sports requiring frequent and forceful hyperextension of the spine place participants at the highest risk. High-risk activities include gymnastics, competitive diving, weightlifting, and the positions of football linemen.
A genetic predisposition can also play a role, as some individuals may be born with a naturally thinner or weaker pars segment. Even with an anatomical variation, the defect is an acquired injury resulting from mechanical stress, not a condition present at birth.
The Link to Vertebral Slippage
The presence of a bilateral pars defect removes the bony restraint that normally prevents a vertebra from shifting forward. When the pars is fractured on both sides, the front part of the vertebra, including the body, pedicles, and disc, is disconnected from the back part. This loss of connection allows the upper vertebra to slide forward over the one directly beneath it.
This forward displacement is a separate condition called spondylolisthesis. Since the pars defect removes the stabilizing connection, the slippage is often progressive and can destabilize the spine. This specific type, caused by bilateral spondylolysis, is known as isthmic spondylolisthesis and is the most common form in younger populations.
Spondylolisthesis is measured using the Meyerding classification, which grades the degree of forward slippage. Symptoms associated with this slippage include generalized lower back pain, muscle tightness—especially in the hamstrings—and a stiff gait. More severe slips can cause nerve root compression, leading to radicular symptoms like pain, numbness, or tingling that radiates down one or both legs (sciatica).
Meyerding Classification of Spondylolisthesis
- Grade 1 slip involves a displacement of up to 25% of the vertebral body.
- Grade 2 ranges from 25% to 50%.
- Grades 3 and 4 represent slippage between 50% and 100%.
- A Grade 5, or spondyloptosis, indicates the vertebra has completely fallen off.
Diagnosis and Treatment Pathways
Diagnosing a bilateral pars defect typically begins with a physical examination that assesses for localized tenderness and pain that worsens with back extension. Imaging studies are then necessary to confirm the diagnosis and assess the extent of the damage. Standard X-rays, including oblique views, can often visualize the defect or the classic “Scottie dog” sign.
Advanced imaging is frequently used to determine the age and healing potential of the defect. A Computed Tomography (CT) scan provides superior bony detail, confirming the presence of a fracture. Magnetic Resonance Imaging (MRI) is valuable for assessing soft tissues, checking for nerve root compression, and detecting early-stage stress reactions or bone marrow edema within the pars.
Treatment for a pars defect starts with conservative management, which is successful for the majority of patients. The initial phase focuses on temporary rest and activity modification to avoid movements that cause hyperextension and aggravate the fracture. A rigid lumbosacral brace may be prescribed for six to twelve weeks to immobilize the area and promote bone healing in cases of acute stress fracture.
Conservative Management
Physical therapy is a fundamental component of recovery, focusing on core stabilization and strengthening the abdominal and back muscles to provide internal support for the spine. Flexibility exercises, particularly for the hamstrings, are also important to reduce tension that pulls on the pelvis and lower back. If conservative treatment fails to relieve persistent pain after six to twelve months, or if there is severe slippage or progressive neurological deficits, surgical intervention may be considered.
Surgical Options
Surgical options include a direct pars repair, which involves stabilizing the defect with screws and bone grafting to preserve spinal motion, often preferred for younger patients. For cases involving significant spondylolisthesis or chronic instability, a spinal fusion procedure may be performed. Fusion permanently joins the affected vertebrae together, stabilizing the segment and preventing further slippage.