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Original Research |
1 Department of Radiology, The University of Iowa Hospital and Clinics, 200 Hawkins Dr., Iowa City, IA 52242.
Received September 9, 2004;
accepted after revision October 28, 2004.
Address correspondence to K. A. Chun
(ka1000{at}catholic.ac.kr).
Abstract
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MATERIALS AND METHODS. A retrospective review of patellar fractures after total knee replacement was performed. Seventeen cases were identified over a 10-year period. Radiographs, radiology reports, medical records, and operative notes were reviewed and analyzed. The clinical analysis included the time to fracture, the patient's symptoms and signs, whether the extensor mechanism was disrupted, and the treatment administered for the fracture. The radiographic analysis included fracture pattern, displacement, patellar position, presence of patellar osteonecrosis, and presence of soft-tissue abnormality.
RESULTS. The incidence of patellar fractures after total knee replacement was 1.14%. The mean time from total knee replacement to patellar fracture was 17.5 months. Nine fractures were asymptomatic and identified on routine follow-up radiographs. The patterns of fracture were diverse: Transverse (n = 6), comminuted (n = 2), vertical (n = 5), and avulsion (n =4) fractures were seen. Thirteen fractures were displaced. Patellar osteonecrosis was suspected in four fractures, and three fractures were associated with disruption of the quadriceps tendons. Thirteen fractures were treated nonoperatively, and four were treated surgically.
CONCLUSION. Patellar fractures are an uncommon, but important, complication after total knee replacement. Radiologists should be familiar with the imaging features of these fractures and their clinical significance.
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Patellar fractures after total knee replacement have been thoroughly discussed in the orthopedics literature. These studies have attempted to evaluate the risk factors for patellar fracture after total knee replacement and to correlate the classification of patellar fractures with treatment and prognosis [9-14]. However, no such studies have appeared in the radiology literature, to our knowledge.
In this article, we report our experience with patellar fractures after total knee replacement, describe their radiographic patterns, explain their clinical significance, and review the literature.
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The average patient age was 71 years (range, 47-84 years) at the time of patellar fracture. Ten patients were women, and five were men. The total knee replacement procedures were performed secondary to osteoarthritis in 16 cases and rheumatoid arthritis in one case. Seven fractures involved the right knee and 10, the left. Bilateral total knee replacements were performed in eight patients, and in two patients, the fractures were bilateral.
We reviewed the radiographs, radiology reports, medical records, and operative notes of all the patients. The clinical data collected included time from total knee replacement to fracture, the patient's symptoms and signs, whether the extensor mechanism was disrupted, and the treatment administered for the fracture. Radiographic interpretations were performed independently by three musculoskeletal radiologists. Disagreements were resolved by majority consensus.
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In the radiographic analyses, the position of the patella was considered normal if its center coincided with the center of the patellar groove of the femoral component, subluxed if its center was overlying the lateral condyle, and dislocated if it was completely displaced from the sulcus. Also considered in the analysis were the fracture pattern (transverse, vertical, comminuted, and avulsion of superior or inferior pole), the location of the fracture (proximal, middle, distal, lateral, and medial), whether the patellar component was loose, and whether the fracture was displaced. Displacement was measured at the point of greatest separation between the fragments, and the fracture was considered displaced if the gap was greater than 2 mm. Radiographic evidence of patellar osteonecrosis was noted if sclerosis, collapse, or fragmentation was present. Follow-up radiographs were evaluated for healing, progression of sclerosis and fragmentation, interval displacement of the fracture fragment, refracture, and patellar instability.
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The fracture patterns were diverse; transverse fractures were seen in six knees (Figs. 1A, and 1B), vertical fractures in five, avulsion fractures in four (Fig. 2), and comminuted fractures in two (Figs. 3A, and 3B). Of the six knees with a transverse fracture, fractures were noted in the proximal third (n = 3), the middle third (n =1), and the distal third (n = 2). The vertical fractures (n = 5) were all laterally based. The avulsion fractures (n = 4) were present in the superior pole of the patella. Thirteen fractures (76%) were displaced, and four were not displaced. The mean initial fracture displacement was 16 mm (range, 3-60 mm). Patellar subluxation was noted in four fractures. In four fractures (24%), there was sclerosis or fragmentation, suggesting patellar osteonecrosis (Figs. 4A, and 4B). Loosening of the patellar component was noted only once. Three fractures (18%) were associated with disruption of the quadriceps tendon, which was confirmed surgically. These fractures were transverse and displaced.
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Follow-up radiographs showed persistence of the fracture with nonunion (n = 6), healing with union of the fracture (n = 5), refracture (n = 1) (Figs. 5A, 5B, and 5C), increased fragmentation with sclerosis (n = 4) (Figs. 6A, and 6B), increased displacement of the fragments (n = 3), patellar subluxation (n = 1), and heterotopic ossification around the patella (n =3).
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The reported incidence of patellar fracture after total knee replacement varies markedly, ranging from 0.5% to 3.8% [1, 5, 12-14, 16]. In our study, the incidence was 1.14%. The differences in the reported incidence correlate with the varying lengths of follow-up time, patient populations, implant designs, and surgical techniques including resurfaced patella [8, 17]. All fractured patellae in our study had been resurfaced.
Most fractures appear to occur in the first few years after total knee replacement [8]. We found that 69% of the fractures in our study group were identified within the first postoperative year. Most patellar fractures occurred in older patients, suggesting that these fractures are more likely to occur in patients with osteoporosis [1]. The average age of our patients was 71 years. Patellar fractures are often discovered incidentally on follow-up radiographs [8]. In our study, 53% of the patellar fractures were asymptomatic.
Risk factors thought to be associated with patellar fractures are numerous and include technical factors, surgical technique, limb alignment, osteonecrosis, and trauma [8, 17-20]. Weakened bone secondary to osteoporosis is an obvious risk factor. Sex and high activity levels are other factors. Unlike other periprosthetic fractures around the knee, patellar fractures appear more frequently in men than in women [13, 14, 20]. The reasons for male predominance are unclear, but may include a higher activity level or higher body weight among men that leads to greater extensor mechanism force and patellofemoral stress [13]. Our series showed no preponderance of male patients because women were more common in the population who underwent total knee replacement in our institution.
Technical factors are important and include patellofemoral malalignment, femoral or tibial component rotational malalignment, patellar devascularization, lateral retinacular release, and excessive or inadequate patellar resection [18]. Patellofemoral complications were once attributed to errors in surgical technique and component design issues [21]. However, recent improvements in surgical technique and instrumentation and in patellofemoral component design have led to an overall decreased incidence of complications [3].
Early designs of total knee replacement implants often did not include patellar resurfacing, and the reported incidence of anterior knee pain associated with these early implants approached 50% [22, 23]. More recent knee implant designs incorporate a patellar flange on the femoral component with patellar resurfacing, which results in decreased incidence of anterior knee pain. Patellar resurfacing has, however, been associated with a variety of patellofemoral complications [5] including fracture. Patellar fractures after total knee replacement occur predominantly in patients with resurfaced patellae [20].
The alignment of the limb, along with the position and rotation of the tibial and femoral components, has important effects on the risk for patellar fracture. Figgie et al. [11] showed a strong correlation between patellar fracture and limb malalignment or malposition of the tibial or femoral components. Implant positions that increase stress on the patella increase the risk of patellar fracture. Excessive patellofemoral malalignment associated with patellar subluxation leads to eccentric loading and contact forces that may contribute to fracture risk [20]. We found patellar subluxation in 24% of the fractures in our study group.
Osteonecrosis also plays an etiologic role in some patellar fractures. Patellar fragmentation and sclerosis of the fragments are presumed to represent osteonecrosis [2]. We found that 24% of the patellar fractures in our study had findings suggestive of osteonecrosis, which is consistent with other reports [13]. The vascular network of the patella is at risk during total knee replacement surgery. Median parapatellar arthrotomy, fat pad removal, and lateral release all contribute to patellar devascularization [8].
Trauma to the patella, either direct or indirect, is associated with patellar fracture. A direct fall on the knee may result in a fracture compromising fixation of the patellar implant or the integrity of the extensor mechanism. Indirect causes might include an eccentric quadriceps muscle contraction associated with a stumble, resulting in an avulsion fracture [18]. We found trauma was a contributing cause in only two of the 17 fractures in our study group. In those two cases, displaced transverse fractures and disruption of the quadriceps tendon were seen. Traumatic fractures usually require an open procedure to reapproximate the fragments [19].
Several classification schemes for patellar fractures after total knee replacement have been proposed [10, 13, 24]. Goldberg et al. [10] developed a classification based on the integrity of the extensor mechanism and the fixation status of the patellar implant. Hozack et al. [24] proposed classifying fractures as undisplaced, displaced, displaced fractures of the distal pole of the patella, and failure of previous treatment. More recently, Ortiguera and Berry [13] proposed a system derived from other widely used periprosthetic fracture classification systems of classifying fractures according to competence of the extensor mechanism, fixation status of the implant, and quality of the remaining patellar bone.
Patellar fractures that are not associated with disruption of the extensor mechanism or loosening of the patellar implant may be treated nonoperatively with good or excellent results. Patellar fractures with extensor mechanism disruption or patellar implant loosening necessitate operative treatment. Hozack et al. [24] showed good results with nonoperative treatments in patients with nondisplaced fractures and intact extensor mechanism. They also reported poor results—even with operative treatment—in patients with extensor mechanism disruption. Patients treated operatively have a very high complication rate. In most cases, patients with fractures of the patella are best treated nonoperatively [14, 18]. In our study, two of the four patients who were treated operatively were symptomatic at the time of follow-up, and one had progression of patellar osteonecrosis and subluxation.
Patellar fractures can be classified into transverse, vertical, or comminuted types. There are also avulsion fractures of the superior or inferior pole. The vertical and transverse fractures may be subdivided further into displaced or nondisplaced fractures [19]. The transverse fracture is usually caused by abnormal patellar tracking and may be associated with patellar instability [25]. The vertical fracture almost invariably passes through the patellar fixation hole, which is usually based laterally. This type of fracture usually heals spontaneously. Comminuted and displaced fractures often are a combination of a transverse or vertical type of fracture.
Occasionally the patellar button may become loose and cause significant anterior pain. At times, the patellar component may be completely disassociated from the underlying patellar bone [19].
In conclusion, patellar fracture is an important complication of total knee replacement. Radiologists should be familiar with the radiologic features of a patellar fracture after total knee replacement and the clinical significance associated with these features.
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