Bipartite Patella: Comprehensive Evaluation and Surgical Management

INTRODUCTION AND EPIDEMIOLOGY

The patella is the largest sesamoid bone in the human body, playing a critical biomechanical role in the extensor mechanism of the knee. A bipartite patella is a congenital anomaly resulting from the failure of one or more secondary ossification centers to fuse with the primary ossification center of the patella.

In the vast majority of cases, a bipartite patella is entirely asymptomatic and is noted incidentally on an anteroposterior (AP) or tunnel tangential radiograph obtained for unrelated reasons. Epidemiological studies indicate that this anatomical variant is present in approximately 2% to 3% of the general population. When present, it occurs bilaterally in approximately 40% of individuals. While the radiographic presence of a bipartite patella has no distinct sex predilection, symptomatic cases are overwhelmingly observed in young male athletes, typically presenting during adolescence or early adulthood.

Understanding the pathoanatomy, biomechanical forces, and appropriate diagnostic algorithms is essential for the orthopedic surgeon, as misdiagnosis can lead to unnecessary interventions, while missed symptomatic cases can result in prolonged, debilitating anterior knee pain.

PATHOANATOMY AND CLASSIFICATION

The patella typically ossifies from a single primary center that appears between 3 and 5 years of age. Secondary ossification centers may appear between 8 and 12 years of age. Normal fusion of these centers occurs during adolescence. When fusion fails, the fragments remain connected by a zone of fibrous or fibrocartilaginous tissue, known as a synchondrosis.

The Saupe Classification

The most commonly used and historically significant classification system for bipartite patella was described by Saupe in 1921. It identifies three distinct types based on the anatomical location of the accessory fragment:

• Type I (5% of cases): Occurs at the inferior pole of the patella. Clinically, this variant must be carefully differentiated from, and may be associated with, Sinding-Larsen-Johansson syndrome (a traction apophysitis of the distal patellar pole).
• Type II (20% of cases): Occurs along the entire lateral border of the patella. This type may be radiographically confused with, or associated with, a nonunion of a longitudinal patellar fracture.
• Type III (75% of cases): The most common type, occurring as an elliptical area in the superolateral portion of the patella. This location is subjected to immense tensile forces from the insertion of the vastus lateralis muscle.

Modern Anatomical Classification

While the Saupe classification remains standard, a more recent and highly descriptive classification scheme considers both the specific location (superolateral or lateral) and the number of fragments (bipartite, tripartite, multipartite).

In a definitive study analyzing 139 knees with this condition, authors found the following distribution:
* 83% were superolateral bipartite.
* 12% were lateral bipartite.
* 4% were superolateral and lateral tripartite.
* 1% were superolateral tripartite.

Clinical Pearl: When evaluating a suspected patellar fracture in a trauma setting, the presence of smooth, well-corticated margins at the fragment interface strongly suggests a bipartite patella rather than an acute fracture. Always obtain contralateral knee radiographs, as the 40% bilateral incidence can aid in confirming the congenital nature of the finding.

BIOMECHANICS AND PATHOPHYSIOLOGY OF PAIN

Pain is unusual in bipartite patella; when present, it is almost exclusively caused by overuse, microtrauma, or a direct traumatic event. The superolateral pole of the patella (Type III) serves as the primary insertion site for the vastus lateralis and the lateral retinaculum.

During repetitive knee flexion and extension—particularly in sports requiring deep squatting, jumping, or rapid deceleration—the vastus lateralis exerts significant tensile traction across the fibrous synchondrosis. This repetitive stress can lead to micro-tearing, localized inflammation, and disruption of the synchondrosis. Alternatively, a direct blow to the anterior knee can acutely disrupt a previously stable, asymptomatic synchondrosis, converting it into a symptomatic lesion.

CLINICAL EVALUATION AND DIAGNOSTIC IMAGING

History and Physical Examination

Patients with a symptomatic bipartite patella typically present with localized anterior knee pain. The pain is exacerbated by activities that increase patellofemoral joint reaction forces, such as climbing stairs, deep squatting, or resisted knee extension.

On physical examination, the hallmark finding is exquisite point tenderness directly over the bipartite fragment (most commonly the superolateral pole). Provocative testing may include resisting knee extension while the examiner palpates the fragment, which often reproduces the patient's symptoms.

Radiographic Evaluation

Standard radiographic evaluation should include weight-bearing AP, true lateral, and skyline (tangential) views of the patellofemoral joint.

A highly specific diagnostic radiographic test can be utilized to determine if the patient's pain is caused by micro-motion and nonunion at the bipartite site. This dynamic assessment involves:
1. Obtaining a standard, non-weight-bearing skyline view.
2. Following this with a skyline view taken with the patient in a squatting, weight-bearing position.

Interpretation: The test result is considered positive if the separation (diastasis) between the main body of the patella and the bipartite fragment is visibly greater in the squatting weight-bearing position than on the normal skyline view. This confirms pathological mobility at the synchondrosis.

Advanced Imaging (MRI and CT)

Magnetic Resonance Imaging (MRI) usually is not strictly necessary for the diagnosis of a bipartite patella. However, in cases of ambiguous anterior knee pain, MRI is highly valuable. It has been shown to identify bone marrow edema within the bipartite fragment and the adjacent primary patellar bone in about half of symptomatic patients. The presence of fluid signal within the synchondrosis on T2-weighted images correlates strongly with clinical symptoms.

Computed Tomography (CT) may be utilized for preoperative planning if Open Reduction and Internal Fixation (ORIF) is being considered, as it provides excellent three-dimensional detail of the fragment size and the articular surface congruity.

NON-OPERATIVE MANAGEMENT

The initial management of a symptomatic bipartite patella is strictly non-operative. A minimum of 3 to 6 months of conservative therapy should be trialed before considering surgical intervention.

• Activity Modification: Cessation of offending activities (jumping, deep squatting).
• Pharmacotherapy: Non-steroidal anti-inflammatory drugs (NSAIDs) to reduce localized inflammation at the synchondrosis.
• Physical Therapy: Focused on hamstring flexibility, quadriceps stretching, and core strengthening. Avoidance of aggressive open-kinetic-chain extension exercises is recommended.
• Immobilization: In acute, severe flares, a brief period (3-4 weeks) of immobilization in a cylinder cast or rigid knee immobilizer may allow the disrupted synchondrosis to heal.
• Injections: Corticosteroid injections directly into the synchondrosis can be diagnostic and therapeutic, though they carry a risk of localized tissue atrophy and should be used sparingly.

SURGICAL MANAGEMENT

Surgical intervention is indicated for patients with persistent, debilitating pain that has failed exhaustive conservative management. The choice of surgical procedure depends primarily on the size of the fragment and the extent of its contribution to the patellofemoral articular surface.

1. Excision of the Fragment

Excision is the most commonly performed procedure for small bipartite fragments that do not comprise a significant portion of the articular surface. This can be performed via an open approach or arthroscopically.

Surgical Technique (Open Excision):
1. Positioning: Supine with a proximal thigh tourniquet.
2. Approach: A longitudinal or oblique incision is made directly over the superolateral pole of the patella.
3. Exposure: The lateral retinaculum and vastus lateralis insertion are carefully incised to expose the fragment.
4. Excision: The fragment is sharply excised using a scalpel to divide the fibrous synchondrosis. Care is taken to avoid damaging the articular cartilage of the main patellar body.
5. Repair: The vastus lateralis and lateral retinaculum must be meticulously repaired to the main body of the patella using heavy non-absorbable sutures through transosseous drill holes or suture anchors to prevent postoperative extensor lag or patellar maltracking.

Surgical Warning: Aggressive drilling or rough handling of an intact lesion during excision can cause fragmentation of the lesion, making complete removal difficult and leaving retained symptomatic bone fragments within the retinaculum.

2. Lateral Retinacular Release

For patients with a small fragment where excision is deemed unnecessary, or as an adjunct to other procedures, a targeted release of the vastus lateralis insertion can be performed. By detaching the vastus lateralis from the superolateral fragment, the tensile forces across the synchondrosis are eliminated, allowing the fibrous tissue to heal and symptoms to resolve.

3. Open Reduction and Internal Fixation (ORIF)

If the bipartite fragment is large and contains a substantial portion of the patellofemoral articular surface, excision may lead to altered patellofemoral contact pressures and early osteoarthritis. In these cases, ORIF is the procedure of choice.

Surgical Technique (ORIF):
1. Approach: A lateral parapatellar approach is utilized to expose the synchondrosis.
2. Preparation: The fibrous tissue within the synchondrosis is meticulously debrided using a curette or high-speed burr until healthy, bleeding cancellous bone is exposed on both the fragment and the main patella.
3. Reduction: The fragment is anatomically reduced, ensuring a perfectly congruent articular surface. This can be verified by palpation or intraoperative arthroscopy.
4. Fixation: Fixation is typically achieved using two parallel cannulated headless compression screws (e.g., 2.4mm or 3.0mm) placed perpendicular to the synchondrosis. Alternatively, a tension band wiring construct may be utilized.
5. Bone Grafting: If a significant gap remains after debridement, local autograft (e.g., from the Gerdy's tubercle or distal femur) may be packed into the defect to promote union.

COMPLICATIONS AND PITFALLS IN SURGICAL MANAGEMENT

Surgical intervention for bipartite patella, particularly internal fixation, carries specific risks that the orthopedic surgeon must anticipate and mitigate.

Hardware Complications

When utilizing metallic screws for ORIF, hardware prominence is a significant concern. Metallic screws that are prominent at the time of surgery, or become prominent as surrounding articular cartilage wears down over time, can severely damage adjacent articular cartilage on the trochlear groove. Abnormally high iatrogenic cartilage damage, hardware loosening, and abrasion of the articular cartilage by hardware are well-documented complications. If hardware prominence occurs, prompt removal usually is recommended once radiographic union is achieved.

Bioabsorbable Fixation Issues

To avoid the need for secondary hardware removal, some surgeons opt for bioabsorbable fixation devices. However, these are not without risk. Absorbable fixation devices have been reported to cause sterile foreign body reactions on occasion, leading to osteolysis and cyst formation. Furthermore, severe damage to adjacent articular cartilage has been attributed to the loosening and failure of bioabsorbable screws that backed out prematurely. In some revision cases, unabsorbed screw heads have been found acting as intraarticular loose bodies, causing mechanical locking and third-body wear.

Nonunion and Delayed Union

Delayed union or nonunion of an internally fixed bipartite lesion is a challenging complication. It is often the result of inadequate debridement of the fibrous synchondrosis or insufficient rigid fixation. A symptomatic nonunion may require an additional procedure, such as revision ORIF with autogenous bone grafting or salvage excision of the fragment.

Complications of Massive Excision

In rare cases where a massive fragment is excised, leaving a significant osteochondral defect on the patellar articular surface, secondary cartilage restoration procedures may be attempted. If osteochondral autograft transfer (OATS) is utilized to fill the defect, surgeons must be aware of donor site morbidity. Fibrocartilage hypertrophy has been reported at osteochondral autograft donor sites (typically the non-weight-bearing periphery of the trochlea), causing subsequent anterior knee pain and occasional mechanical locking that required secondary arthroscopic trimming of the hypertrophic fibrocartilage.

POSTOPERATIVE REHABILITATION

Rehabilitation protocols vary significantly based on the surgical procedure performed.

Post-Excision Protocol:
* Weeks 0-2: Weight-bearing as tolerated in a knee immobilizer locked in extension to protect the retinacular repair. Gentle passive range of motion (ROM) is initiated (0-30 degrees).
* Weeks 2-6: Progressive increase in ROM. Discontinue immobilizer once straight leg raise is achieved without an extensor lag. Initiate closed-kinetic-chain exercises.
* Weeks 6-12: Advance strengthening. Return to sports is typically permitted between 8 to 12 weeks once full strength and ROM are restored.

Post-ORIF Protocol:
* Weeks 0-4: Touch-down weight-bearing with the knee locked in extension. ROM is strictly limited to 0-45 degrees to prevent excessive tension on the fixation.
* Weeks 4-8: Progress to full weight-bearing. Gradually advance ROM to 90 degrees. Radiographs are obtained to assess for interval healing and hardware position.
* Weeks 8-16: Once radiographic union is confirmed, aggressive strengthening begins. Return to high-impact sports may be delayed until 4 to 6 months postoperatively to ensure complete bony consolidation of the synchondrosis.

CONCLUSION

While bipartite patella is predominantly a benign, asymptomatic congenital variant, it remains an important differential diagnosis in the evaluation of anterior knee pain. A thorough understanding of the anatomical classifications, particularly the high prevalence of superolateral lesions, guides accurate diagnosis. Dynamic radiographic evaluation and MRI are excellent adjuncts for confirming the source of pathology. When conservative measures fail, surgical management—ranging from fragment excision to meticulous internal fixation—yields excellent clinical outcomes, provided the surgeon navigates the specific biomechanical challenges and hardware-related pitfalls associated with the patellofemoral joint.

📚 Medical References

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