In lower-limb pathomechanical patient populations, one of the greatest impediments to energy-efficient ambulation is the development of a genu recurvatum deformity. When this deviation occurs, the thigh and lower limb segments move posteriorly in direct opposition to the anterior advancement of the proximal body mass over the fixed distal base of support. Genu recurvatum usually is an acquired deformity secondary to changes of the distal skeletal joint alignments and compensatory movement patterns. The alignment of the distal base of support determines the load bearing and functional motion available to the proximal knee joint during ambulation and can serve either to promote or disrupt the required sagittal plane limb advancement.
This article describes three distinct pathomechanical profiles of posterior knee joint deviation in relation to the primary planes of motion. The distinction between an initial hyperextension moment and an acquired recurvatum deformity is presented in terms of predictability, prevention and correction of joint deformation. Gait training and orthotic design criteria are discussed as they relate to a comprehensive rehabilitation program to maximize both structural and functional outcomes for patients with lower-limb instabilities and deformities.
Key Words: Genu Recurvatum; Pathomechanics; Deformity.
Genu recurvatum is a commonly acquired deformity found in many rehabilitation populations with both musculoskeletal and upper motor neuron pathologies (1). Skeletal segment deviations are accompanied by soft-tissue laxities of the posterior, posterior-medial or posterior-lateral joint structures and are identified throughout the loading phase of gait. Decreased step length, stride length, velocity, and cadence are primary functional gait deviations associated with this deformity (2). Increased lateral trunk displacement and increased energy costs also are likely to be noted.
Numerous causes of genu recurvatum are cited in the literature and include plantarflexion contracture, spasticity of the triceps surae, quadriceps weakness, limb-length discrepancy, and hip extensor weakness (2-5). The actual genesis of this type of deformity also may be influenced by any postural adjustment developed to achieve limb stability and prevent anterior knee collapse.
This article explores the development of acquired genu recurvatum as it relates to the mechanical lever profiles of the distal base of support. When misaligned levers of the base are compounded with neuromuscular involvement such as extensor tone patterns, weak quadriceps or posterior calf muscle group, or timing disruption of muscle contractions, the knee is subjected to abnormal moments throughout the stance phase of the gait cycle. An initial hyperextension moment will precede the actual joint deformity and is observed as a momentary posterior deviation of the knee joint occurring at or around midstance (6). This gait deviation disrupts sagittal plane limb and body advancement, thereby necessitating greater effort to maintain forward momentum. The duration of this event is relatively short, and the integrity of the soft tissue structures are challenged but intact at this point.
Unopposed, a posteriorly directed hyperextension moment may progress into severe deformation. Genu recurvatum deformity refers to a sustained posterior deviation of the knee joint occurring throughout the loading period, from initial contact through forward progression (6). The magnitude of the posteriorly directed ground reaction forces are significantly increased in combination with joint displacement, and the duration of the event is prolonged. Permanent damage to soft tissue structures results in greater posterior deviation, which serves to increase the moments of force produced at the knee by the resultant lengthening of the anterior forefoot base lever. The specific type of genu recurvatum acquired is dependent primarily on the configuration of the distal triplanar alignments of the midtarsal, subtalar and talocrural joints. In addition, the overall limb orientation relative to the hypothetical line of progression of the center of mass of the body influences the mechanical effects of distal base levers.
The knee joint is the largest joint of the human skeletal system; biomechanically, it is one of the most complex joints. The asymmetrical design of the condylar surfaces of the femur contributes to the complexity of joint motion as well as to the integrity of joint stability during load bearing. With six degrees of freedom, the motion of the knee joint is a combination of gliding and angular movement. The anatomical axis shifts orientation in all three perpendicular planes of reference during both swing and stance phases, although the primary joint motion occurs in the sagittal plane.
Static load-bearing evaluation of the lower limb in the "normal" model reveals a noncollinear load transfer through the femur. Distally, load transfer through the tibia is collinear, as anatomical and mechanical axes are the same. The orientation of the knee joint axis is nearly perpendicular to the line of progression, with noted internal and external rotation occurring during initial loading and from midstance forward progression, respectively.
Habitually loaded in full extension, the coronal and sagittal plane alignments contribute to joint stability and provide continuous support for the proximal body mass during both single- and double-limb support phases. Displacement forces are resisted by the structural integrity of the joint, supporting soft tissue structures (i.e., ligaments and muscles), menisci, and compression forces.
Inman et al. (7) proposed that knee flexion during stance and the combined motions of the knee and ankle were two critical elements necessary to translate the center of mass in an energy-efficient manner. Knee motion is essential for the transfer of potential to kinetic energy from weight acceptance through forward progression. In a normal gait cycle, the knee progresses from approximately 0 to 15 degrees of flexion during the loading period, extends to 0 degrees by midstance and maintains a fully extended alignment until the period of double support when the knee flexes again in preparation for swing phase.
Initial stance-phase knee flexion contributes to the dissipation of forces encountered during loading while midstance knee extension creates the distal stability needed for the smooth transfer of proximal weight over the supporting limb. In this way, forward progression of the body mass through a space is maintained and energy costs are minimized at self-selected walking speeds.
Based on the mechanical loading patterns of the foot and ankle complex, three distinct types of genu recurvatum are identified. The two most common types involve significant transverse plane alignment characterized by excessive external or internal rotation of the talocrural joint axis, with secondary knee joint deviation and axis misalignments. The third type of genu recurvatum is uniplanar and presents with sagittal plane involvement in which compensatory talocrural joint plantarflexion accompanies the primary knee joint deviation. These three presentations are referred to as external rotary deformity recurvatum (ERD recurvatum), internal rotary deformity recurvatum (IRD recurvatum), and nonrotary deformity recurvatum (NRD recurvatum), respectively (8).
When a tone-induced equinovarus positioning of the foot and ankle complex is sustained during both swing and stance phases, an initial hyperextension moment will be produced at the knee joint. The distal base has acquired a load-bearing posture that now sustains the forefoot in a position of adduction, supination, and plantarflexion. As a result, the anterior-lateral lever function of the forefoot has been decreased, allowing excessive lateral knee joint deviation. The calcaneus is sustained in a position of varus and dorsiflexion, which serves to decrease the effectiveness of the posterior-lateral heel lever. Convergence of the talonavicular and calcaneocuboid joints (midtarsal adduction) rigidifies the foot and prevents the normal pronation moment from occurring during initial stance. This type of triplanar foot deformity has been identified as an external rotary deformity (ERD) (9).
The loss of mechanical base levers, specifically the anterior-lateral forefoot and posterior-lateral hindfoot, allows the talus to displace in a posterior-lateral direction with an external rotary torque (Figure 1) . This external torque is immediately transferred to the talocrural joint as well as proximally to the knee joint in a closed kinetic chain. As talocrural joint plantarflexion and spasticity often accompany this neurologic presentation, the knee joint is now prominently displaced in the posterior-lateral direction with an external orientation of the knee joint axis.
Typically, this can be evidenced in a patient recovering from a recent stroke, where an extensor tone pattern maintains an equinovarus posturing. Initial loading of the base occurs with toe-heel contact sustained, plantarflexion, posterior-lateral ankle joint displacement with external rotation and posterior-lateral knee joint deviation with external rotation. The hyperextension and genu varum deviation is most pronounced around midstance; obvious disruptions to forward momentum can be easily observed.
The long-term implications of this type of gait disruption include increased stress to the soft tissue structures and the acquisition of permanent damage and deformation. The mechanical factors contributing to the development of an ERD recurvatum include the nonyielding and rigid anterior forefoot lever, loss of posterior-lateral hindfoot levers, posterior-lateral displacement of the talus and ankle joint with external rotation, sustained talocrural joint plantarflexion, and anterior trunk lean. As the integrity of the soft tissue structures at the knee are overcome, the posterior knee joint deviation continues in a direction that is perpendicular to the joint axis alignment. As a result, the ERD recurvatum develops with a valgum component. With continued posterior-medial displacement of the knee joint, the mechanical effect of the rigid anterior forefoot lever increases and acts to promote even greater knee joint displacement.
Figures 2a and 2b present sagittal and coronal plane views of a patient's left limb with an acquired ERD recurvatum and the corresponding X-rays. As previously described, a genu recurvatum angulation presents with a slight genu valgum alignment.
An internal rotary deformity (IRD) (9) is identified with the decrease in external rotation alignment of the talocrural axis, as with a "hyperpronated" foot. The initial knee joint deviation will follow the displacement of the talus in the anterior and medial direction, with internal rotation (Figure 3 ). When compounded by neurological deficits such as weak quadriceps or a weak gastrocnemius-soleus complex, most patients will posturally hyperextend to shift the load bearing line of force anterior to the knee joint and produce a stable limb. Over time, soft tissue damage will occur, and an IRD recurvatum deformity will develop.
The mechanical factors contributing to this deformity are likely to produce a less marked recurvatum deformity, primarily due to the loss of an effective anterior-medial forefoot lever. In this pathomechanical presentation, the midtarsal joint is abducted, pronated, and dorsiflexed against the plantarflexed and everted hindfoot. The divergence of the talonavicular and calcaneocuboid joints creates a supple foot, with little or no anterior forefoot lever in place to promote continued posterior knee displacement. In this profile, the talocrural and knee joint axes are internally rotated; therefore, the posterior knee deviation proceeds with a lateral component producing a genu varum deviation.
Figures 4a and 4b reveal the distinct differences in limb alignment when compared to previous recurvatum. An IRD recurvatum is determined by an increase in the talocalcaneal angle, measurable genu recurvatum, and genu varum alignments.
It is rare that recurvatum deformities present as single-plane involvements. Still, traumatic injuries or unique circumstances can produce a primary sagittal plane deviation of the knee joint with compensatory talocrural joint plantarflexion (Figure 5) . In this case, the foot and ankle complex is not specifically affected, and the integrity of the anterior-lateral and anterior-medial forefoot levers remain intact. While the hindfoot remains in a vertical position during loading, the posterior lever is decreased as the proximal knee joint deviation progresses posteriorly. Load-bearing stresses to the posterior soft tissue structures are increased with each degree of continued deformation, as the moment arm incrementally increases as well. The single-plane involvement does not involve excessive transverse plane deviations of the transmalleolar or knee joint axes and therefore is referred to as a third type of genu recurvatum, or nonrotary deformity (NRD) recurvatum.
As shown in Figures 6a and 6b , an NRD recurvatum is a uniplanar deformity. Significant sagittal plane recurvatum is noted with no coronal plane deviation. The talocalcaneal alignment and angle are within normal limits.
Many orthotic designs have been proposed to address the problem of genu recurvatum. Different types of knee cages attempt to prevent posterior knee displacement but do not address the distal deviations of the base of support or provide stance-phase stability for limbs with accompanying motor weakness. As a result, patients will continue to "override" these external restraints as the orthoses can produce inherent instability and insecurity in the patient's gait. The most effective designs for genu recurvatum occur when the base levers of the foot are restored (10), stance-phase stability is ensured against anterior knee collapse, and effective therapy and training have developed an efficient pattern of distal-limb segment sequencing and proximal trunk control.
Orthotic designs for ERD and IRD recurvatum deformities require realignment of the deviated forefoot and hindfoot segments. The talus and ankle joint then are stabilized over the distal foot, with effective base levers for dynamic load transfer. A limitation to plantarflexion prevents the development of posterior knee deviation when accompanied by an effective heel-fulcrum strategy during initial loading. Any compromise to proximal joint stability by allowing distal plantarflexion at initial contact will increase the length of the anterior base lever (i.e., the linear distance from the knee center to the metatarsal heads) and diminish functional ambulation. Temporary heel lifts and adjustable ankle joints ensure the distal limb is aligned in up to 3 degrees to 5 degrees of knee flexion during stance phase when a plantarflexion contracture is present. This promotes sagittal plane limb and center of mass advancement.
Stance-phase stability must be provided by the orthosis to prevent anterior knee collapse. This stability is best achieved with a rigid, nonyielding orthotic footplate and a limitation to excessive dorsiflexion range from footflat through forward progression. Flexibility or excessive range of motion allowed in the orthotic design produces instability and fear of falling for the patient with insufficient motor strengths. When this occurs, patients will develop compensatory strategies such as decreased step length, external rotation of the entire limb or failure to transfer load to the forefoot to ensure limb stability. Development of an energy-efficient gait pattern is compromised when the orthosis does not provide structural stability and fails to enhance functional outcomes (5).
Orthotic prescription rationale is based on the degree of deformity, degree of correctability, potential functional abilities of the patient, and anticipated gait training protocols. Generally, knee orthoses are not indicated for triplanar and/or severe forms of genu recurvatum as they do not realign the distal base levers or compensate for weakness of the quadriceps or posterior calf group without a locking knee mechanism. Locking the knee joint, which increases metabolic costs, serves to essentially lengthen the limb during swing phase. Ankle-foot orthosis applications are suggested up to 20 degrees of genu recurvatum, when the mechanical potential for limb stability has been verified during ambulation in a corrective fiberglass cast and physical therapy gait training is available. Knee-ankle-foot orthoses are recommended for sagittal plane deformities greater than 20 degrees, in limbs with coronal plane deviations greater than 10 degrees, and for patients unable to receive physical therapy gait training.
Three significant phases of gait should be addressed when training the patient with a genu recurvatum deformity: initial contact, midstance and forward progression. Initial contact must occur with a heel fulcrum strategy and continued knee flexion until footflat is achieved. Plantarflexion range must be blocked in preparation for this phase of gait or the weightbearing line from the center of mass will remain anterior to the knee joint, forcing the knee posteriorly. This initial heel contact and anterior knee advancement are difficult concepts for some patients to accept until they have developed confidence in the stance-phase stability of the orthotic system. Minimizing posterior pelvic rotation and hip extension helps to advance the knee forward in the sagittal plane from heelstrike to footflat to maintain the momentum developed during swing and sustain anterior advancement of the COM.
Once footflat is achieved, the proximal body mass now progresses over the fixed distal limb. The knee gradually extends during the period and should reach a terminal angulation of about 3 degrees to 5 degrees of flexion. By midstance, the center of mass is proceeding anteriorly over the base and limitation to talocrural joint dorsiflexion maintains proximal knee joint stability. Uncontrolled or excessive dorsiflexion range will result in increased knee flexion and limb instability where posterior calf group weakness exists. Absolute stability during single-limb support is required from the orthotic design and will serve to decrease loading forces of the hands and axillary regions for those patients dependent on external support. Flexibility and excessive motion will be perceived by the patient as instability and prevent the development of an effective terminal loading strategy during the next phase of gait.
During forward progression, a terminal loading strategy of the lower limb must be developed to maximize sagittal plane progression of the contralateral swing limb and minimize energy expenditures (11). A rigid orthotic design will resist the forces encountered at this stage and promote sagittal plane limb and body advancements (12). The patient is taught to "load the toes" when excessive dorsiflexion is effectively blocked. Forward progression continues smoothly until contralateral heelstrike and swing phase are initiated. Excessive dorsiflexion range of motion must be presented at this stage to prevent 1) potential knee instability, 2) excessive distal displacement of the center of mass if the knee is allowed to flex and 3) a delay in the initiation of swing and contralateral load transfer.
Gait training procedures can be enhanced by static weight shift and balancing exercises. Initial training sessions often occur within the security of parallel bars, allowing the patient to focus on proper sequencing and control of the limb. Proximal trunk and pelvis control can be improved when distal stability of the limb is provided by a mechanically effective orthosis (Figure 7) . Transferring gait training tasks to longer parallel bars or external walking aids allows the development of increased velocity at self-selected walking speeds once the patient has gained confidence in the support of the orthosis and a new gait pattern. The need for external walking aids often decreases as the patient's ambulation skills improve and the amount of attention focused on the activity declines and becomes habitual.
Genu recurvatum is an extremely complex and debilitating deformity of the lower limb. The mechanical components of genu recurvatum most often are determined by the base of support when the knee joint deviation is acquired secondary to lower-limb dysfunction. The mechanical effectiveness of force-transferring or force-generating levers of the base of support must be addressed to maximize knee joint stability and functioning. Once mechanical disruptions to forward progression have been addressed, efficient movement patterns then can be superimposed. Early identification and treatment of knee joint deviations are indicated for prevention of continued ambulatory decline.