Fracture Complications after Anterior Cruciate Ligament Reconstruction

Anterior cruciate ligament (ACL) reconstruction is one of the most frequently performed operative procedures, with more than 100,000 reconstructions performed annually in the United States alone. Autogenous bone–patellar tendon–bone (BPTB) is one of the most frequently used graft choices by orthopaedic surgeons in the United States, Canada, and Europe. This procedure includes creation of large bony defects in the tibia, femur, and patella for graft harvesting and fixation. The effects of bony defects on bone strength have become a major concern in orthopaedic trauma surgery, and their relevance in the development of postoperative fracture after ACL reconstruction is increasingly recognized. Complications have been reported to occur in 1.8%–24% of ACL reconstructions. Serious complications after ACL reconstruction include arthrofibrosis, donor site pain, patella tendinitis, patellar tendon rupture, and avascular necrosis of the femoral condyles. Fracture following ACL reconstruction presents a devastating complication that may involve the tibia, patella, or femur.

Femur Fracture

Femur fracture following ACL reconstruction has been reported in isolated cases as a result of distal femoral bone defects created for extra-articular fixation of a GoreTex prosthetic graft, a ligament augmentation device, iliotibial band tenodesis, double-bundle reconstruction, or femoral postfixation. Supracondylar femur fracture after arthroscopic ACL reconstruction without intraoperative complications or use of supplemental fixation has also been reported. Fracture of the femoral diaphysis has also been described after ACL reconstruction, and it is caused by multiple perforations of the Beath pin trough’s femoral metaphyseal-diaphyseal junction. There are nine reports in the literature of femoral fractures at the sites of graft fixation. Although femur fractures are reported with increasing frequency after ACL reconstruction, this complication is likely underreported, and its exact incidence is not known.

Physical examination in patients with this complication always produces marked tenderness, muscular guarding, bony crepitation, and a large effusion. Plain radiographs and computed tomography (CT) scans are helpful to identify the fracture pattern and will often show that the fracture occurred through the intraosseous tunnel created in the posterior distal femur. CT scans may demonstrate increased bone tunnel diameter ( Fig. 135.1 ).

Several factors predispose the anatomical area of the femoral tunnel to developing a distal femur fracture after arthroscopic ACL reconstruction. The presence of the large femoral tunnel likely acts as a predisposing factor due to the localized stress-rising effect of the bony defect. This effect results from a concentration of local stresses around the femoral defect and reduced energy-absorbing capacity from the decreased amount of bone available to withstand the applied load. Because bone with stress concentration behaves in a more brittle fashion, the increased local stresses can reach the ultimate stress of the bone at much lower applied loads. Depending on the geometry of the defect, strength reductions of 20%–90% may occur. Double-bundle ACL reconstruction surgery may increase the chances of peri-ACL femur fractures due to a greater stress concentration effect from the presence of multiple femoral tunnels and multiple cortical violations (Heng). Back wall blowout could happen, because during extreme knee flexion, the socket will be parallel to the true back wall of the intercondylar notch. This could be avoided by conservative selection of the aimer offset. Insertion of allogenic or autogenous bone graft into the defect, such as in BPTB ligament reconstruction, has not been shown to significantly change the mechanical weakening of the bone. The combination of greater localized stresses and decreased load-absorbing capacity predisposes the area of the defect to failure. Aside from the bony defect, additional stress concentration in the distal femur results from the change in the bony moment of inertia due to the acute change of sagittal, axial, and coronal geometry of the posterior condylar flare and intercondylar notch. The bony geometry of the distal femur has been found to play a critical role in the structural properties and prediction of the fracture load. Geometric analysis of the distal femur has shown the thinnest cortical shell to be in the posterior aspect of the distal femur, therefore predicting the lowest fracture load in the anatomical region of the femoral tunnel.

Decreased bone mineral density (BMD) of up to 20% has been observed after knee ligament injury and may also contribute to increased fracture risk after ACL reconstruction due to decreased bending strength in the distal femur. Ten studies investigating the influence of ACL injury and reconstruction on bone mineral density (BMD) were included in a systematic review. All 10 studies that reported on BMD or bone content did not return to normal levels after ACL injury or reconstruction; premorbid bone integrity is not reestablished after ACL reconstruction, even when accelerated rehabilitation is performed. Bone bruising of the lateral femoral condyle, which is frequently associated with ACL rupture, may also compromise the biomechanical properties in the lateral femoral condyle and predispose to earlier failure of the bruised bone. When the area of the bony defect is subjected to tensile stress, as with extension trauma to the knee, the load strength of the already vulnerable posterior distal femur is even further reduced. However, because the bone in this anatomical region is predominantly under compression, the likelihood of fracture development and crack propagation is decreased. This may explain why femur fracture does not occur more frequently after arthroscopic ACL reconstruction.

Because bony remodeling has been shown to decrease stress concentration around bony defects after 8–12 weeks, this would be expected to decrease the predisposition for femur fracture after ACL reconstruction. However, bone tunnel healing of the femoral tunnel has been shown to be delayed by the exposure to biological factors from the joint. A previous report demonstrating fracture through the femoral tunnel 2 years after ACL reconstruction suggests that the stress concentration effect of the femoral tunnel continues for a prolonged period after surgery.

Bone tunnel enlargement after ACL reconstruction is well documented and occurs in as many as 68% of cases after ACL reconstruction. The etiology of this clinical phenomenon is not completely understood, but it is thought to be related to a combination of multiple biological and mechanical factors such as type of graft, graft tunnel motion, stress deprivation of bone within the tunnel wall, improper graft tunnel placement, aggressive rehabilitation, and synovial fluid leakage within the bone tunnel have been considered. A better understanding of the clinical relevance of bone tunnel enlargement is still evolving. Previous experimental studies have shown that the breaking strength of bone decreases in direct proportion to the size of a bony defect. Based on these findings, enlargement of the femoral tunnel may have clinical relevance for the development of supracondylar femur fracture after ACL reconstruction by further decreasing the mechanical fracture resistance. Bone tunnel enlargement has also been suggested to increase the risk for tibial plateau fracture after ACL reconstruction. Given the frequency of ACL reconstruction and the high incidence of bone tunnel enlargement, the potential predisposing effect of this phenomenon for fracture of the distal femur needs to be further examined.

During ACL reconstruction, a tunnel is drilled into the distal femur for subsequent graft fixation. Femoral tunnel placement is performed arthroscopically in accordance with recent technique recommendations. To optimize graft positioning, the femoral tunnel is placed as far posterior as possible, while carefully avoiding disruption of the posterior cortex. This is commonly achieved by the use of a femoral tunnel placement guide with built-in offset that maintains a 1- to 2-mm-thick posterior cortical rim. Disruption of the posterior cortex can result from posterior placement of the femoral tunnel. This complication is different than fracture through the femoral tunnel. However, it should be carefully avoided because it may facilitate development of a fracture of the lateral femoral condyle ( Fig. 135.2 ). ACL reconstruction using computer-assisted navigation systems for tunnel placement or using a two-incision technique for ACL reconstruction may be able to reduce the risk of this complication.

Anatomical open reduction is critical to avoid premature arthritis and may also be able to maintain the graft in the isometric position. Fracture fixation by interfragmentary screws, supracondylar blade plates, dynamic compression plates, and intramedullary nails have all been described after femur fracture following ACL reconstruction. Permanent loss of knee motion has been described in some cases after distal femoral fracture fixation. Early open reduction and internal fixation using condylar locking plates provide effective fracture fixation with limited soft tissue dissection and reduced postoperative morbidity, and may allow for graft retention. A proactive treatment approach facilitates early recovery, full range of motion, excellent subjective knee rating, high functional outcome scores, and return to pivoting sports. Following anatomical fracture fixation, intraoperative stability testing may reveal a functional ACL graft without the need for revision ACL reconstruction. If anatomical fracture fixation does not maintain graft function, revision ACL reconstruction may be performed at the time of fracture fixation or at a later time.

Patella Fracture

Patella fracture represents a devastating complication following ACL reconstruction using BPTB autograft and has been reported with an incidence of 0.2%–2.3%. Patella fractures after ACL reconstruction are more frequently observed in women and older patients. These fractures can occur intraoperatively or postoperatively. Intraoperative fractures are very rare and most often result from technical errors such as harvesting too large a bone block. More frequently, injuries occur between 5 and 12 weeks postoperatively. These fractures in the early postoperative period are considered a major complication because they may interfere with graft remodeling and produce significant chondral damage and persistent anterior knee pain. Most fractures are transverse and occur at the proximal margin of the defect created during bone block harvesting ( Fig. 135.3 ). Patella fracture following ACL reconstruction using bone–patella–bone autograft has been associated with direct or indirect trauma. The transverse fracture pattern usually results from indirect injuries to the patella, such as rapid eccentric contractions of the quadriceps muscle. Stellate and Y-type fracture patterns have been described after direct-impact injuries to the weakened patella. Longitudinal patella fractures may also occur from vertical troughs created by past-pointing with the saw blade. Patella fractures can be displaced or nondisplaced, and may or may not be symptomatic. Most fractures cause a disturbance of the extensor mechanism and are easily diagnosed. However, silent nondisplaced patella fractures have been reported after ACL reconstruction as a cause of chronic anterior knee pain and long-term functional limitation.

Multiple factors have been suggested to contribute to the development of this complication. Decreased vascularity to the central portion of the patella after graft harvest has been suggested to contribute to the risk for postoperative patellar fracture. The intraosseous blood supply of the patella is composed of the midpatellar, polar, and quadriceps tendon system. The disruption of these vessels during graft harvest can impair healing of the harvest site and may even affect the remaining normal bone.

In knee flexion, the patella is subject to posterior forces along the superior and inferior poles, and an opposing anterior force when the posterior surface of the patella contacts the femur. This three-point bending force on the patella has been hypothesized to be the force that acts on the weakened patella, resulting in many of the indirect patella fractures that occur when the knee is flexed.

Biomechanical studies have shown that the anterior cortex of the patella has the highest load resistance and that superior transverse cuts during graft harvest can reduce patellar resistance by 30%–40%. The transverse bone cut has been shown to average more than 13 mm in width because of subtle motion of both the patella and the cutting instrument. Attention to surgical technique with smaller transverse bone cuts can help minimize the weakening effect on the anterior patellar cortex. Drill holes at the corners of the planned osteotomy can also be helpful to prevent past-pointing and undesired propagation of the bone defect beyond the outline of the osteotomy. Several authors have pointed out that the dimensions of the grafted bone plug should not exceed 9–10 mm in width. Using a 7-mm-wide sagittal blade and angling the blade can reduce the width of the true traversal cut. In addition to the width of the bone defect, its length and depth are important. Graft length of less than 50%–66% of the patellar length is recommended with minimum graft length of 20 mm. Similarly, depth of the graft should not exceed one-third of the measured depth of the patella.

Harvesting the BPTB graft acts as a significant stress riser on the patellar bone. This stress-rising effect is directly correlated with the size of the bone defect. Tapered bone defects cause less patellar stress concentration compared with square or trapezoidal defects, but are still associated with significantly higher stress concentration than in the normal patella. Taking the larger trapezoidal bone plug from the tibia and a triangular plug from the patella has been suggested as a technique to reduce the patellar defect size and stress concentration. Packing the patella defect with cancellous bone grafts has been recommended by many authors. Bone grafting has been shown to decrease the stress-rising effect and can help normalize the strength and resistance of the harvest site. Grafting the bony defect is particularly recommended for graft sites wider than 10–12 mm or deeper than 6 mm. In addition to graft size and shape, the grafting technique is also important. Use of a circular oscillating saw can create lower stresses on the corners, remove smaller grafts, and create a rounded bottom of the trough.

It has been theorized that some patella fractures after ACL reconstruction result from a weakened patella and abnormal patellar tracking due to a deconditioned quadriceps, which in turn increases the patellofemoral contact stresses. Early quadriceps conditioning and proprioceptive exercises decrease abnormal patellar positioning and tracking.

Improved quadriceps strength and function may also help restore normal gait pattern and thereby prevent direct fractures from falls or from sudden uncoordinated muscle contraction.

Nondisplaced fractures can be treated nonoperatively with rigid knee bracing. However, some authors advocate surgical fixation for all fractures because it restores the extensor mechanism and allows for immediate motion and rapid return to knee rehabilitation. We have successfully used cannulated screw fixation with figure-eight Fiberwire augmentation through the cannulated screws for these nondisplaced fractures. With appropriate treatment, minimal residual long-term consequences have been observed after patella fractures following ACL reconstruction with functional outcomes, comparable to patients without fracture complications.

Patella Fracture

Patella fracture represents a devastating complication following ACL reconstruction using BPTB autograft and has been reported with an incidence of 0.2%–2.3%. Patella fractures after ACL reconstruction are more frequently observed in women and older patients. These fractures can occur intraoperatively or postoperatively. Intraoperative fractures are very rare and most often result from technical errors such as harvesting too large a bone block. More frequently, injuries occur between 5 and 12 weeks postoperatively. These fractures in the early postoperative period are considered a major complication because they may interfere with graft remodeling and produce significant chondral damage and persistent anterior knee pain. Most fractures are transverse and occur at the proximal margin of the defect created during bone block harvesting ( Fig. 135.3 ). Patella fracture following ACL reconstruction using bone–patella–bone autograft has been associated with direct or indirect trauma. The transverse fracture pattern usually results from indirect injuries to the patella, such as rapid eccentric contractions of the quadriceps muscle. Stellate and Y-type fracture patterns have been described after direct-impact injuries to the weakened patella. Longitudinal patella fractures may also occur from vertical troughs created by past-pointing with the saw blade. Patella fractures can be displaced or nondisplaced, and may or may not be symptomatic. Most fractures cause a disturbance of the extensor mechanism and are easily diagnosed. However, silent nondisplaced patella fractures have been reported after ACL reconstruction as a cause of chronic anterior knee pain and long-term functional limitation.