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PubMed This is a summary of 20 peer-reviewed journal articles Updated
Orthopedics

Why Does MO Cause Bone Deformities & Uneven Leg Length?

At a Glance

Multiple osteochondromas (MO) cause bone deformities because the bone bumps act like anchors near active growth plates, restricting normal growth. This uneven growth leads to forearm bowing, knock-knees, and uneven leg lengths. These changes stop progressing once a child reaches skeletal maturity.

In this answer

4 sections

01

Why the Forearm Bows (Ulnar Shortening)

02

Leg Length Discrepancies and Ankle Angling

03

Knock-Knees (Genu Valgum)

04

What Can Be Done?

It can be startling to notice changes in the shape of your child’s arms or legs, but forearm bowing and leg length differences are very common, classic features of multiple osteochondromas (MO) [1]. The short answer is that the bone bumps (osteochondromas) develop near the growth plates and act like physical anchors, slowing down bone growth in certain areas [1][2].

Imagine a young tree trying to grow upward, but a heavy wire is tied to one side of the trunk. As the tree grows, the restricted side cannot keep up with the free side, causing the entire trunk to curve. In a similar way, when an osteochondroma forms at the growth plate, it “tethers” the bone, leading to bowing, shortening, or angled growth like knock-knees [1][3][2].

Because these deformities are driven by active growth plates, it is reassuring to know that the bowing and lengthening differences will stop progressing once your child reaches skeletal maturity (when they finish growing and their growth plates close) [4][1].

Why the Forearm Bows (Ulnar Shortening)

The forearm is made up of two parallel bones: the ulna (on the pinky side) and the radius (on the thumb side). In MO, osteochondromas frequently form at the bottom of the ulna near the wrist, slowing its growth [5][6]. Because the two bones are connected by ligaments, problems arise when they grow at different speeds.

While the ulna’s growth is stunted by the osteochondroma, the radius continues to grow at a more normal pace [7][8]. Because the radius has nowhere straight to go, it begins to bend and bow outward to accommodate its extra length [7][8][9]. Over time, this uneven growth can push the top of the radius out of its normal position at the elbow, a condition called radial head dislocation [7][9].

How it feels: This is not just a cosmetic change. This bowing and dislocation can physically block the joint, making it difficult for the child to fully rotate their forearm (turning the palm up or down, known as supination and pronation) [10][7].

Leg Length Discrepancies and Ankle Angling

The bones of the legs grow the fastest, particularly around the knee. When osteochondromas develop near these highly active growth plates, they can interfere with normal, straight bone lengthening [1][11].

If the bumps affect the growth plates in one leg more than the other, that leg will grow more slowly, eventually leading to a noticeable difference in leg length [1][3][11]. This might cause a noticeable limp or back pain.

A similar tethering “race” happens in the lower leg. The lower leg has two bones: the tibia (shinbone) and the fibula. Osteochondromas often stunt the fibula, and as the tibia keeps growing, it causes the ankle to angle outward (ankle valgus) [5][1].

Knock-Knees (Genu Valgum)

Another common result of uneven growth is “knock-knees” (medically known as genu valgum). This happens when an osteochondroma acts as a tether on just one side of the knee’s growth plate [2][12]. If the outer edge of the growth plate is restricted while the inner edge continues to grow normally, the leg will begin to angle inward toward the other knee [1][3][2].

What Can Be Done?

Understanding why these changes happen is the first step, but it is also important to know that orthopedic specialists have a step-by-step approach to managing these deformities, starting with the least invasive options:

  • Monitoring: Regular X-rays (typically every 6 to 12 months) help doctors track the growth plates, measure differences in bone length, and check the alignment of joints [13][14].
  • Shoe Lifts: For mild to moderate leg length discrepancies, a simple insert or lift added to the child’s shoe can level the hips, improve their gait, and prevent back pain without any surgery [5][15].
  • Early Removal: Sometimes, carefully removing an osteochondroma that is acting as a tether can relieve the tension [2][12]. However, simply removing the bump does not always guarantee the growth plate will correct its trajectory on its own, so it is often combined with other procedures [16].
  • Guided Growth: For angled deformities like knock-knees, doctors can perform a minor procedure (such as placing a small plate or staple) on the faster-growing side of the growth plate. This temporarily slows it down, allowing the slower side to catch up and straighten the leg [17][18].
  • Bone Lengthening: In severe cases where the ulna becomes extremely short or a leg length difference is very large, complex surgical techniques can be used to gradually lengthen the bone [19][20]. Because this is a major intervention, it is generally reserved for situations where the deformity causes significant functional problems.

Common questions in this guide

Why do multiple osteochondromas cause bone deformities?
Osteochondromas often develop near active growth plates and act like physical anchors. This "tethers" the bone, slowing down growth on one side and causing the bone to bow, shorten, or angle as the rest of the bone continues trying to grow.
Will the bone bowing and leg length differences keep getting worse?
These deformities are driven by active growth plates. Reassuringly, once your child reaches skeletal maturity and their growth plates close, the bowing and leg length differences will stop progressing.
What causes forearm bowing in multiple osteochondromas?
Bumps often form on the ulna bone near the wrist, stunting its growth. The parallel radius bone continues to grow normally but has nowhere straight to go, causing it to bend outward and sometimes dislocate at the elbow.
How are leg length discrepancies treated without surgery?
For mild to moderate differences in leg length, doctors often recommend adding a simple shoe lift. This insert helps level the hips, improves the child's walking gait, and can prevent back pain without the need for surgery.
Can removing the osteochondroma fix the bone angle?
Carefully removing a bump that is tethering a growth plate can relieve tension. However, simply removing it does not guarantee the bone will straighten on its own, so it is often combined with procedures like guided growth.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Based on their current bone age, how much remaining growth potential does my child have?
  2. 2.How much of a leg length difference is acceptable before we need to intervene?
  3. 3.Could my child benefit from a simple shoe lift to help with their gait or back alignment?
  4. 4.Are there specific activities or sports my child should avoid if they have forearm bowing or a leg length difference?
  5. 5.If we surgically remove a bump that is tethering a growth plate, will my child also need guided growth to correct the angle?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

Related questions

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References

References (20)
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This page provides educational information about bone growth in multiple osteochondromas. Always consult a pediatric orthopedic specialist for an accurate assessment of your child's specific deformities and treatment options.

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