To our best knowledge, this study was the first study to investigate the corrections between the angles of hyperextension and the gait parameters, joint angles, kinetic parameters. Also, the correlations between the angles of knee hypertension and meniscus volume, and the water content of meniscus were investigated. Surprisingly, the results in this study found low correlations between the angles of knee hypertension and gait parameters, joint angles, and kinetic parameters. However, the results found that higher knee hyperextension indeed had a strong-to-moderate negative correlations between the angles of hyperextension and medial/lateral meniscus volume in paretic legs.
Longer time since hemiplegia led to larger angles of knee hyperextension
It has been reported that approximate half of stroke survivors experienced the knee hyperextension. Importantly, this knee hyperextension may be caused by muscle weakness and the impairment of proprioceptive sensors ; furthermore, this knee hyperextension causes pain and limit the patients’ movement in daily life activities . However, in past decades, it has been widely investigated the symptoms of the knee hyperextension in stroke survivors. For instance, this knee hyperextension generates the knee extensor moment to prevent from falls during stance phase. However, it has never been shown the correlation between the time since hemiplegia and the angles of knee hyperextension in the previous literature. In the current study, a positive correlation was observed between the time since hemiplegia and the angles of knee hyperextension. In other words, longer time since suffering hemiplegia, larger angles of knee hyperextension could be gained. Therefore, it is worth mentioning this correlation because the best treatment exist for knee hyperextension in stroke patients at the current moment is the Ankle–foot orthoses. However, a study also shows that in patients with severe knee hyperextension, the Ankle–Foot orthoses might not be effective . Using Knee–Ankle–Foot orthosis might an appropriate alternative . However, the drawbacks of the Knee–Ankle–Foot could delay the recovery of normal gait and increase the spasticity of the gait in stroke survivors . In such a case, based on the observation in this study, to identify the time since hemiplegia can be an initial screen to identify whether the Ankle–Foot or Knee–Ankle–Foot orthoses.
Larger angles of knee hyperextension led to more tears in medial/lateral meniscus
The medial and lateral menisci anatomically cover the superior aspect of tibia. The lateral meniscus is more circular than the medial meniscus; however, the medial meniscus is more crescent-like than the lateral meniscus. The roles of menisci are to bear the stress across the knee during standing or walking. Importantly, these menisci are to prevent knee hyperextension during walking. However, when putting too much pressure on the knee joint, the knee joint is forced to extend further than the healthy range of motion resulting in potential tears of the ligaments in stroke patients. There is a close correlation between ankle, knee and hip joint . After stroke, triceps surae spasm or lower extremity muscles weakness cause hip, knee and ankle dysfunction, resulting in asymmetrical gait, presenting with significant reduction of support time, speed, stride length and step length, and significant increase of stride width of the hemiplegic extremity. During knee flexion and extension activities after stroke, decreased muscle strength and flexibility combined with the sensory impairment especially the proprioception of the hemiplegic extremity caused the meniscus cannot move with the tibial plateau, making it vulnerable to the injury by the compression of the femoral condyle and tibial plateau . This might be the rationales that a negative correlation between the angles of hyperextension and the meniscus volume was found in both paretic legs in the current study. Specifically, due to the shape of medial meniscus, the medial meniscus in general suffers three times more stress than the lateral meniscus during walking . In the current study, a negative correlation between the angles of hyperextension and the medial meniscus volume was observed, indicating that higher hyperextension might lead to more tears in the medial meniscus not only in the paretic leg but also in the non-paretic leg. This result might raise a concern that the knee hyperextension might need to be seriously treated as soon as possible.
Larger angles of knee hyperextension decreased water content of the lateral meniscus in the non-paretic leg but increased water content of the medial meniscus
Firstly, the average water contents were 52.85%, 53.89%, 51.98% and 53.68% of the lateral meniscus—Non-Paretic, lateral meniscus—Paretic Leg, medial meniscus—Non-Paretic, and medial meniscus—Paretic Leg, respectively. These mean values were much lower than normal healthy controls (77 ± 3.3%) [17, 18]. It was not a surprising result, indicating no matter knee hyperextension existed or not, the level of water content apparently decreased by the meniscus degradation due to the stroke . The interesting results were that larger angles of knee hyperextension had a tendency to decrease water content of the lateral meniscus than in the non-paretic leg but had a tendency to increase water content of the medial meniscus in the paretic leg. In our previous study, genu recurvatum in stroke patients with hemiplegia also cause changes in the moisture content of the knee cartilage . This could be explained by Warnecke et al., study that as increasing degrees of degenerations of medial meniscus in the paretic leg, the water content significantly increases and further leads to worsen viscoelastic properties inside the knee . This phenomenon might reduce the water content of lateral meniscus in the non-paretic leg due to the compensated mechanism.
The angles of knee hyperextension and gait parameters had a no or low correlation
The alternations of gait parameters due to the quadriceps weakness, quadriceps spasticity, calf muscles spasticity have been linked to the knee hyperextension. Surprisingly, in the current study, a low or no correlation between angles of knee hyperextension and step length, cadence, or walking speed. Similar results have found in a review with 44 relevant studies, involving 2658 patients with stroke, concludes that stroke survivors who receive treadmill are not likely to improve their ability in their step length, cadence, and stance phase . However, using joint angles and kinetic parameters might be better options to identify the knee hyperextension than spatial–temporal gait parameters. Importantly, in the current study, larger knee hyperextension in the paretic leg induced larger hip flexion in non-paretic leg but not the ankle joint to provide a compensation for balance. This result inferred that for stroke patients, the leading joint was the proximal joint because these proximal joints could generate larger power to maintain balance and also moved the body forward.