Introduction
Bone contusions of the knee refer to bone injuries seen on magnetic resonance imaging (MRI) and are also referred to as bone bruising and bone marrow edema. These lesions likely depict hemorrhage, edema, or infarction resulting from trabecular microfractures. The prevalence of bone contusions has been reported to be as high as 80% in anterior cruciate ligament (ACL)-injured patients, and is primarily found on the lateral femoral condyle (LFC) and lateral tibial plateau (LTP), as demonstrated in Fig. 105.1 . LTP bone contusions tend to occur in the posterior area of the tibia, whereas the location of LFC bone contusions tends to vary and has been observed in the anterior, middle, and posterior areas of the femur. Medial bone contusions have also been observed, but are less common than lateral contusions. Determinants of bone contusions include younger age, female sex, noncontact injuries, acute injuries, injuries that do not involve jumping, and unstable ACL ruptures.
Diminished signal intensity on T1-weighted MRI as well as increased signal intensity on T2-weighted MRI indicate a bone contusion. Bone contusions can be used as a secondary sign for diagnosing ACL ruptures in cases where the ACL may be obscured on MRI due to extensive edema or fibrous scarring. Time to resolution of bone contusions on MRI varies broadly from 3 weeks to 2 years. Variation in healing time may be due to the severity of the bone contusion and the initial injury, associated pathology, and management of the injury. Moreover, one study reported that 34% of patients with acute ACL injuries developed new bone contusions within 2 years of their initial injury, which the authors attributed to microtrauma during intense sports, bone remodeling, or a biologic mechanism in the bone–cartilage junction.
Bone contusions represent the footprint of the mechanism of the ACL injury and most commonly result from pivot-shift and hyperextension injuries. Anterior tibial subluxation occurs during pivot-shift injuries, which involve rapid deceleration and simultaneous direction change, and leads to impaction of the LFC on the posterolateral region of the LTP. Pivot-shift injuries result in bone contusions on the posterior region of the LTP, middle of the LFC, and occasionally on the posterior MTP. Hyperextension injuries are caused either by direct force to the anterior tibia while the foot is planted or by indirect force during forceful kicking. ‘Kissing’ bone contusions result from the impact of the anterior tibial plateau hitting the anterior femoral condyle.
Bone contusions are also significant because they may be associated with intra-articular injuries, pain, and outcomes following ACL rupture. The focus of this chapter is to provide a summary of the current evidence regarding these associations.
Intra-Articular Injuries
Cartilage Injuries
Cartilage injuries have been reported to occur concomitantly with bone contusions and ACL ruptures. Several studies have examined changes in bone contusions and the overlying cartilage in patients with acute ACL tears using T 1 ρ MRI, which unlike standard MRI has the capability of quantifying changes in the extracellular matrix. These studies found T 1 ρ signal to be significantly greater in the cartilage overlying the bone contusion versus the surrounding cartilage on the LTP ( Fig. 105.2 ). Theologis et al. also found significantly increased T 1 ρ signal in the overlying cartilage versus the surrounding cartilage on the MTP and MFC, and alternatively, an increase in T 1 ρ signal in the surrounding cartilage versus the overlying cartilage on the LFC. The latter finding may be attributed to the fact that T 1 ρ signals are generally lower in weight-bearing regions, which is mainly where LFC bone contusions occurred in this study. There was also a positive correlation between volume of LTP bone contusions and T 1 ρ signal ( r = .74, P < .05). Furthermore, research has shown that arthroscopically confirmed chondral lesions on the LFC are associated with LFC bone contusions seen on preoperative MRI in ACL-injured patients. Nawata et al. found that 21% of patients had matching chondral lesions and bone contusions on the LFC.
Other studies have examined the histology of cartilage overlying bone contusions. Johnson et al. obtained biopsies of articular cartilage and subchondral bone surrounding LFC bone contusions in 10 patients prior to undergoing ACL reconstruction (ACLR). All biopsies had some degree of histological abnormality, including degeneration of chondrocytes and loss of proteoglycan. Two of the biopsies also revealed necrosis of osteophytes in the subchondral bone and empty lacunae. Fang et al. measured the distribution and amount of cartilage oligomeric matrix protein (COMP), which mediates the relationship between chondrocytes and the extracellular matrix, in synovial fluid and tissue samples in 13 patients with lateral bone contusions at the time of ACLR. It was found that COMP levels varied by layer of cartilage at bone contusion sites, such that the highest concentrations were found in the superficial layer and the lowest concentrations were found in the deep layer. Mean COMP levels in synovial fluid were significantly greater in injured knees versus contralateral knees (26.3 vs. 2.6 μg/mL, P < .05). Articular cartilage at bone contusion sites also had increased apoptotic chondrocytes, empty osteocyte lacunae in subchondral bone, and decreased proteoglycan in the extracellular matrix. In addition, glycosaminoglycan (GAG) levels have been measured in the femoral cartilage of ACL-injured patients with bone contusions and compared with healthy controls. GAG is a component of a proteoglycan, aggrecan, found in the extracellular matrix in articular cartilage, and was measured with delayed gadolinium-enhanced MRI (dGEMRIC), as well as synovial fluid concentration. In ACL-injured patients there was a loss of GAG in the LFC and MFC, the regions where the most and least bone contusions occurred, respectively. There was also a positive correlation between GAG measured by dGEMRIC and in synovial fluid ( r = .49, P = .02).
Articular cartilage degeneration may progress after ACLR and lead to later development of knee osteoarthritis (OA), although little is known about how preoperative bone contusions might contribute to this process. Potter et al. conducted a longitudinal study of cartilage injury in 42 knees with acute, isolated ACL ruptures treated either nonoperatively or operatively. Bone contusions on the LTP were significantly associated with cartilage loss at years 1 ( P = .001), 2 ( P = .008), and 3 ( P = .039), whereas bone contusions on the LFC were significantly associated with cartilage loss at years 1 ( P = .03) and 2 ( P = .025). To date, no studies have examined the association between severity and location of bone contusions and long-term risk of knee OA after ACLR.
Meniscus Tears
Meniscus tears have been observed laterally in 35%–75% and medially in 10%–40% of ACL-injured patients with bone contusions. Several studies have found a significant association between meniscus tears and bone contusions in ACL-injured patients, while others have observed no association. Bisson et al. found that LFC (odds ratio [OR] = 2.57; 95% confidence interval [CI]: 1.04, 6.32) and LTP (OR = 3.13; 95% CI: 1.06, 9.23) bone contusions were associated with lateral meniscus tears in patients undergoing ACLR after adjusting for age, sex, body mass index, mechanism of injury, type of injury, and chondral injuries. Moderate (OR = 8.14; 95% CI: 1.93, 34.27) and severe (OR = 15.30; 95% CI: 2.34, 100.10) LTP bone contusions were also associated with medial meniscus tears. Yoon et al. found significantly more medial meniscal injuries in ACL-injured patients with lateral or medial bone contusions. Vincken et al. also found that patients with ACL and meniscus tears were more likely to have bone contusions present than absent in any compartment of the knee.
Bone contusions could be associated with meniscal tears due to increased energy and force at the time of injury, which may result in greater intra-articular injury. Meniscal injury is a risk factor for later development of knee OA. Meniscectomy or meniscal repair at the time of ACLR has been shown to be associated with articular cartilage degeneration at 2–4 years postoperatively; however, it is unknown if the presence or severity of bone contusions increases the risk of articular cartilage degeneration in this cohort.
Intra-Articular Injuries
Cartilage Injuries
Cartilage injuries have been reported to occur concomitantly with bone contusions and ACL ruptures. Several studies have examined changes in bone contusions and the overlying cartilage in patients with acute ACL tears using T 1 ρ MRI, which unlike standard MRI has the capability of quantifying changes in the extracellular matrix. These studies found T 1 ρ signal to be significantly greater in the cartilage overlying the bone contusion versus the surrounding cartilage on the LTP ( Fig. 105.2 ). Theologis et al. also found significantly increased T 1 ρ signal in the overlying cartilage versus the surrounding cartilage on the MTP and MFC, and alternatively, an increase in T 1 ρ signal in the surrounding cartilage versus the overlying cartilage on the LFC. The latter finding may be attributed to the fact that T 1 ρ signals are generally lower in weight-bearing regions, which is mainly where LFC bone contusions occurred in this study. There was also a positive correlation between volume of LTP bone contusions and T 1 ρ signal ( r = .74, P < .05). Furthermore, research has shown that arthroscopically confirmed chondral lesions on the LFC are associated with LFC bone contusions seen on preoperative MRI in ACL-injured patients. Nawata et al. found that 21% of patients had matching chondral lesions and bone contusions on the LFC.
Other studies have examined the histology of cartilage overlying bone contusions. Johnson et al. obtained biopsies of articular cartilage and subchondral bone surrounding LFC bone contusions in 10 patients prior to undergoing ACL reconstruction (ACLR). All biopsies had some degree of histological abnormality, including degeneration of chondrocytes and loss of proteoglycan. Two of the biopsies also revealed necrosis of osteophytes in the subchondral bone and empty lacunae. Fang et al. measured the distribution and amount of cartilage oligomeric matrix protein (COMP), which mediates the relationship between chondrocytes and the extracellular matrix, in synovial fluid and tissue samples in 13 patients with lateral bone contusions at the time of ACLR. It was found that COMP levels varied by layer of cartilage at bone contusion sites, such that the highest concentrations were found in the superficial layer and the lowest concentrations were found in the deep layer. Mean COMP levels in synovial fluid were significantly greater in injured knees versus contralateral knees (26.3 vs. 2.6 μg/mL, P < .05). Articular cartilage at bone contusion sites also had increased apoptotic chondrocytes, empty osteocyte lacunae in subchondral bone, and decreased proteoglycan in the extracellular matrix. In addition, glycosaminoglycan (GAG) levels have been measured in the femoral cartilage of ACL-injured patients with bone contusions and compared with healthy controls. GAG is a component of a proteoglycan, aggrecan, found in the extracellular matrix in articular cartilage, and was measured with delayed gadolinium-enhanced MRI (dGEMRIC), as well as synovial fluid concentration. In ACL-injured patients there was a loss of GAG in the LFC and MFC, the regions where the most and least bone contusions occurred, respectively. There was also a positive correlation between GAG measured by dGEMRIC and in synovial fluid ( r = .49, P = .02).
Articular cartilage degeneration may progress after ACLR and lead to later development of knee osteoarthritis (OA), although little is known about how preoperative bone contusions might contribute to this process. Potter et al. conducted a longitudinal study of cartilage injury in 42 knees with acute, isolated ACL ruptures treated either nonoperatively or operatively. Bone contusions on the LTP were significantly associated with cartilage loss at years 1 ( P = .001), 2 ( P = .008), and 3 ( P = .039), whereas bone contusions on the LFC were significantly associated with cartilage loss at years 1 ( P = .03) and 2 ( P = .025). To date, no studies have examined the association between severity and location of bone contusions and long-term risk of knee OA after ACLR.
Meniscus Tears
Meniscus tears have been observed laterally in 35%–75% and medially in 10%–40% of ACL-injured patients with bone contusions. Several studies have found a significant association between meniscus tears and bone contusions in ACL-injured patients, while others have observed no association. Bisson et al. found that LFC (odds ratio [OR] = 2.57; 95% confidence interval [CI]: 1.04, 6.32) and LTP (OR = 3.13; 95% CI: 1.06, 9.23) bone contusions were associated with lateral meniscus tears in patients undergoing ACLR after adjusting for age, sex, body mass index, mechanism of injury, type of injury, and chondral injuries. Moderate (OR = 8.14; 95% CI: 1.93, 34.27) and severe (OR = 15.30; 95% CI: 2.34, 100.10) LTP bone contusions were also associated with medial meniscus tears. Yoon et al. found significantly more medial meniscal injuries in ACL-injured patients with lateral or medial bone contusions. Vincken et al. also found that patients with ACL and meniscus tears were more likely to have bone contusions present than absent in any compartment of the knee.
Bone contusions could be associated with meniscal tears due to increased energy and force at the time of injury, which may result in greater intra-articular injury. Meniscal injury is a risk factor for later development of knee OA. Meniscectomy or meniscal repair at the time of ACLR has been shown to be associated with articular cartilage degeneration at 2–4 years postoperatively; however, it is unknown if the presence or severity of bone contusions increases the risk of articular cartilage degeneration in this cohort.
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