Knock Knee Deformity

Knock knee is an angular deformity of the knees with the knees pointing inwards on standing straight. The patient has their knees touching together but ankles wide apart when standing straight. Knock knees deformity or genu valgum is usually a part of normal growth and development in children. It is different from genu varum or bow legs where the knees are wide apart but the ankles touch on standing straight.

Knock knees in adults may be a result of the residual knock knee deformity, trauma to the knee joint, and arthritis. Usually, in the older age groups, arthritis, such as rheumatoid arthritis may cause knock knee deformity. The deformity increases with time in adults as every step with a mal-aligned knee joint further damages the outer side of the knee.

The knee joint is formed by the lower end of the thigh bone and the upper end of the shinbone. A common concern for parents, knock knees and bow legs are quite common in growing children. 

There is a normal outward deviation of the knees (bow-legs) from birth until one year of age. The bow leg deformity usually corrects by itself to a neutral axis by the age of two years. The deformity then changes to knock knees with the knees angled inwards as the child grows. 

In young children, the knock knee deformity increases till the age of three and a half years. The deformity usually reaches a normal angle which is slight knock knees by the age of 7-8 years. The deformity may point to an underlying bone disease if it doesn’t correct by itself or appears later in life.

Symptoms

Persisting knock knee deformity may present with symptoms such as knee pain, limp, instability of the knee, or difficulty walking. Pain is present usually as a result of excessive pressure on the outer side of the knees. The pain may be absent initially and only appear a few years later. The pain is generally diffuse and exacerbated with activity. 

The child or adult may walk with a limp which is characteristically circumducting. The severity of limping increases with the increase in the intensity of the knee pain. Pain and limping may severely limit the patient’s physical activities.

With knock knee deformity, the kneecap is unable to properly sit in the cavity of the lower end of the thigh bone. The kneecap, therefore, is at an increased risk of dislocation. Patellofemoral mal-tracking may result in chronic pain in front of the knee. 

Visibly, there is a gap in between the ankles as the patient straightens the lower limbs. The gap can be even 3 inches or greater causing a cosmetic deformity as well. The outer side of the knee joint remains under extra pressure. This may lead to meniscal tears or osteoarthritis in the future.

In advanced cases, the lower end of the thigh bone and the upper part of the shinbone are rotated outwardly. The lower part of the shinbone rotates inwards to compensate for the rotation in the upper part. This may result in flat foot and outwardly rotated foot deformity. 

Causes

Most commonly in the majority of cases, it is a physiological part of normal bone growth. Persistent knock knee deformity after the normal age may be a result of underlying bone disease.

Rickets is a common cause of persistent knock knee deformity. It is due to deficiency of vitamin D, calcium and phosphate in the body. Most commonly nutritional in nature but may result from an underlying disease of kidneys. Rickets may also be inherited and causes weakening or softening of the bones. 

Knock knees may also result from developmental syndromes such as Morquio syndrome. Injuries to the growth plate of the knee joint may also result in deformity. The injury may be physical trauma, fractures,  infections or loss of blood supply to the growth plate. The deformity is usually unilateral or single-sided. 

Rarely, knock knee may be present because of underlying benign tumors such as osteochondroma. Obesity may mimic knock knee deformity but there is no underlying bone deformity. Obesity may increase the severity of deformity when already present.

Knock knees deformity in adults may result from osteoarthritis or rheumatoid arthritis. The cartilage damage on the outer side of the knee joint leads to decreased joint space and knock knee deformity. The deformity gets significantly worse with time. The constant pressure on the outer side of the knee joint leads to progressive worsening.

Diagnosis

On doctor visits, the attending physician will take a detailed history regarding the progression of deformity, any congenital diseases, and birth history. Physical examination may involve a detailed evaluation of the gait of the patient.

The doctor may further take measurements of the leg and assessment of any leg length discrepancy. The range of motion and rotational movements of the joints are assessed. Hips, ankles, and feet are evaluated to rule out any deformity in them. 

Knock knee deformity could lead to significant laxity on the inner side of the knee joint. There may be tightening of the structures on the outer side of the knee joint. 

Blood tests may be required to rule out any deficiency of vitamin D, calcium or phosphate. The doctor may further request imaging studies in the form of X-rays to assess the degrees of deformity. 

Management

Most children with physiological knock knees only require close monitoring and frequent evaluations throughout the growing period. The deformity usually disappears as the child reaches the age of 7 or 8 years. If the bone and joint deformity is a result of underlying medical illness, the treatment is first directed to correct the medical illness. 

Nonsurgical methods of treatment are required only after the age of 7-8 years when the deformity doesn’t correct by itself. Leg braces may be used to align the knees in the mechanical axis of the legs. Further, arch pads or specially designed orthopedic shoes may be utilized. In obese and overweight patients, weight loss may be advised.

Surgical management is usually required for only a small percent of children. Guided growth procedures are done to modulate the growth of the knees. Small pins or metal implants are used to tether the inner part of the knee designed to stop the growth. The outer part of the knees grows normally correcting the deformity. 

The implants are placed at a strategic time and need to be removed at a set time for accurate correction of the deformity. The procedure is not a major surgery and patients return to their activities early. Osteotomies are done after the end of the growth spurt in older children.

Knock knee deformity in adults secondary to osteoarthritis or rheumatoid arthritis usually require total knee replacement. Total knee replacement promises correction of deformity, pain-free mobility, and faster rehabilitation. 

Early stages of mild deformity may be managed with high tibial osteotomy. The osteotomy literally means cutting of the bone. The surgery involves opening a small wedge in the outer part of the lower end of the thigh bone. 

Osteotomy may also be performed on the inner side where a wedge of bone is removed. The osteotomy surgeries require longer rehabilitation and protected weight-bearing.

Total knee replacement offers an excellent treatment for knock knee deformity secondary to arthritis. Occasionally, this may be complicated by excessive tightening of structures on the outer side of the knee. During the surgery, the outer side of the knee joint is carefully stretched/released to prevent damage to the common peroneal nerve.

The common peroneal nerve passes along the outer side just below the knee joint. Excessive release in the form of stretching may damage the nerve leading to paralysis of key muscles of the foot. In cases where there may be bone loss in the outer compartment of the knee, bone grafts may be utilized during the surgery. 

Total knee replacement offers an excellent option in complete correction of the deformity. There is also a relief of pain from arthritis. Successful knee replacement surgeries last for many years or a patients lifespan.