A benign condition characterized by multiple osteochondromas (bony growths) found on the surfaces of bones, often at the sites of tendon insertions.
More Information on Multiple Hereditary Exostoses
MHE is a genetic autosomal dominant disorder of most commonly the EXT1 and EXT 2 genes. The prevalence of MHE is 1 in 50,000. MHE is associated with characteristic progressive skeletal deformities of the extremities and shortening of one or both sides leading to limb length discrepancy and short stature. Two bone segments such as the lower leg or forearm are at greater risk of problems due to either osteochondromas from one or both bones impinging on or deforming the other bone or a primary issue of altered growth causing one bone to grow at a faster or slower rate. Osteochondromas can affect joint motion due to impingement of an osteochondroma with the opposite side of the joint or subluxation/dislocation related to deformity, impingement and incongruity. Osteochondromas can also cause nerve or vessel entrapment and/or compression including of the spinal cord and nerve roots. Tethering of tendons and muscles can lead to locking or restriction of range of motion of joints as well as pain.
A defect in the EXT1 or EXT 2 gene causing a problem in the formation of Heparin Sulfatase.
MHE can be diagnosed on clinical examination, x-rays and for confirmation genetic testing can be performed although 15% of patients with MHE will have “normal” genetics with the current technology.
The patient as a whole needs to be treated, listened to and managed. There is no reason for an individual with MHE to suffer from chronic nerve and/or joint pain. Since MHE is a single gene disorder affecting the biosynthesis of a specific molecule, heparin sulphate (HS), medical treatment should be possible. HS impacts many signaling pathways in bone and cartilage including FGF, Wnts, Ihh, BMP, TGFβ, and SH2. Each of these pathways are possible targets for drug development. Since BMP signaling is critical for cartilage and bone formation, it may be the ideal therapeutic target. Palovarotene (PVO) is a retinoic acid receptor γ (RARγ) selective agonist. PVO has been identified as an effective treatment for MHE and we are performing a randomized trial to prove this..
MHE can affect any of the bones in the lower extremity (pelvis, femur, tibia, foot bones). The indications for surgical treatment are numerous and are related to age, osteochondroma size and location, effects on joint range of motion, deformity and stability, alignment and length discrepancy, and nerve entrapment and pain.
Children with MHE will often require numerous osteochondroma (OC) removal. It is important to limit the number of surgeries so as to minimize the interruption of childhood. To this end the indication for removal of OCs may be softened to include ones that are not yet symptomatic but are expected to become so. This way a combination of removal of numerous OC’s, is often combined with other reconstructive procedures including osteotomies, hemi-epiphysiodesis, lengthening, nerve decompression, etc. One should avoid repeated single OC surgery that interrupts childhood. When both upper and lower limb problems need to be addressed, one should consider doing all of the upper limbs at one time and all of the lower limbs at another time.
- The Hip: Osteochondromas of the hip are common and these may impinge on the hip movement. The hip also may start to dislocate. Both of these issues can be managed often surgically by removing the tumors and at times by redirecting the ball or socket of the hip to maintain the normal roundness of the hip and prevent arthritis. MHE affects both sides of the hip joint, although the femur is more often primarily affected which then leads to secondary dysplasia in the acetabulum. Computerized tomography as well as MRI are useful modalities to evaluate the hip. A three dimensional CT study is the most helpful in understanding the location of the offending OCs. These are most often located on the femoral neck but can also be present on the outside or inside of the acetabulum.OCs in this region lead to limitation of joint motion and pain, both due to impingement. They can protrude posteriorly and entrap or irritate the sciatic nerve. The femoral neck tends to grow into a valgus position. In some cases the neck shaft angle can reach 180°. One should look for a break in Shenton’s line as well as an enlargement of the medial joint space, both indicators of hip joint subluxation. Subluxation may be related to one or more of the following: coxa valga, medial femoral OCs, and intra-acetabular OC. It is preferable to treat the hip in MHE before the acetabulum becomes dysplastic to avoid the need for a pelvic osteotomy. Coxa valga should be treated by a varus inter-trochanteric osteotomy at the level of the lesser trochanter to minimize the amount of medial translation required. The center of rotation of angulation (CORA) of the coxa valga is at the level of the base of the lesser trochanter. Therefore an osteotomy through or above the lesser trochanter is almost at the same level as the CORA while an osteotomy below this level requires medial translation to maintain the colinearity of the piriformis fossa with the mid-diaphysis of the femur. If there are non-impinging OCs of the calcar region, these will move away from the pelvis with the varus osteotomy. If there are associated impinging OCs of the femoral neck or calcar, a safe surgical dislocation approach combined with the varus osteotomy is carried out. Limitation of hip range of motion, femuro-acetabular impingement and associated nerve irritation due to posterior Osteochondromas of the proximal femur are all treated by the Ganz safe surgical dislocation method. Postoperatively, an assessment and decision needs to be made whether there is sufficient coverage of the femoral head. If the acetabulum is too dysplastic then a peri-acetabular osteotomy should be performed. In children under the age of 8, it is preferable to avoid the hip dislocation so as not to disrupt the greater trochanteric apophysis. In most of these children there is marked coxa valga but limited medial OCs. A surgical dislocation can be deferred to an older age. Isolated varus osteotomy is performed.
- The Knee: Frontal plane alignment is the most common problem at the knee. There is a tendency of the tibia to grow into valgus. This is often related to osteochondromatous involvement of the upper fibula suggesting that there is lateral tethering of growth of the tibia. The simplest way to treat this problem is using a medial hemi-epiphysiodesis plate. To confirm that the deformity is coming from the tibia and not the femur, the Paley Malalignment test should be performed. This involves measuring the lateral distal femoral angle (LDFA) and the medial proximal tibial angle (MPTA). Based on these one can determine whether the valgus is coming from the femur or tibia or both. If there is significant OC involvement of the proximal fibula, the peroneal nerve should be decompressed at its two tunnels and the OCs of the proximal fibula removed. This includes an anterior and lateral compartment transverse fasciotomy. Sagittal plane malalignment is also common but less clinically relevant. Hyperextension deformity of the distal femur is often present and rarely needs surgery. In contrast flexion deformity of the distal femur leads to fixed flexion of the knee and requires osteotomy. Osteochondromas of the distal femur and proximal tibia are often large, symptomatic and prominent. Medial tibial OCs can lead to entrapment of the pes anserinus tendon and catching of the knee joint, during flexion and extension. The femoral vessels are often in contact or displaced by medial distal femur OCs. Lateral distal femoral OCs can entrap the iliotibial band. Anterior distal femoral OCs can affect the patella and quadriceps mechanism. Posterior distal femur OCs can limit knee flexion range of motion due to impingement with the posterior tibia with knee flexion. This will resolve itself in most cases as the OC grows away from the joint. Posterior proximal tibial OC can displace the vessels or nerves depending on its location. When it is postero-lateral it can collide with the fibula. Kissing tibio-fibular OCs are common both proximally and distally. Removal of femoral and tibial OCs requires careful extensile anatomic exposure with special attention to the neurovascular structures at risk. It is often safer to remove an OC piecemeal rather than try and resect the entire exostosis as one piece. The cartilage cap should always be removed in its entirety since it contains the growth plate for enlargement of the OC. To prevent bleeding bone way can be pressed onto the base of the resected bone.
- The Ankle: Valgus deformity at the ankle is very common in MHE. This deformity may manifest in four stages.
- Type 1: Valgus plafond (sometimes the distal epiphysis may be wedged), no talar shift, fibula at station.
- Type 2: Valgus plafond, lateral talar shift, fibula migrated proximal.
- Type 3: V-shaped distal tibial epiphysis, lateral talar shift, fibula migrated proximal.
Treatment of Type 1 ankle: These cases can be easily treated by medial distal tibial hemi-epiphysiodesis in the growing child or by supramalleolar osteotomy of the distal tibia and fibula in the skeletally mature individual.
Treatment of Type 2 ankle: This requires a procedure called the SHORDT (SHortening Osteotomy Realignment Distal Tibia). The distal fibula is proximally migrated in these cases. This position is referred to as the fibula not at station. To restore the fibula to station, the tibia is shortened relative to the fibula. This involves releasing the anterior and posterior distal tibio-fibular ligaments combined with a varus supramalleolar osteotomy with shortening of the tibia by the amount of proximal migration of the fibula.
Treatment of Type 3 ankle: These cases require a procedure called the Pentagon Osteotomy. The distal fibula is proximally migrated with the talus following it and tilting into valgus. This leads to valgus deformation of the lateral distal tibial epiphysis. A valgus angle develops between the medial and lateral plafond. The talus articulates only with the lateral plafond. There is a space between the talus and the medial malleolus. The distal tibiofibular ligaments are released. The supramalleolar osteotomy is made in a V shaped fashion with each limb of the V parallel to its section of the tibial plafond. A second osteotomy is made more proximally for shortening of the tibia to restore the fibula to station. Finally, an incomplete intra-articular osteotomy is added through the apex of the V to level the plafond. In growing children this osteotomy is through the distal tibial physis. Bone wax is inserted in the opening wedge space to prevent a physeal bridge in the growing child.
Associated with the valgus ankle deformity in types there are often interosseous osteochondromas between the distal tibia and fibula coming from one or both bones. The anatomy of this is best studied using a 3D CT scan. These osteochondromas must be resected at the same time as the rest of the treatment. With types 2 and 3 an anterior incision is made to allow such resection and osteotomy of the distal tibia. An anterior plate and screws are used for fixation.
In order to correct the valgus of the ankle the subtalar joint must at least be able to move into its neutral position. To determine this, examine whether the heel can be put into the same amount of valgus as the valgus of the tibial plafond. If the heel cannot evert enough then there is a fixed subtalar varus contracture which will limit how much varus correction can be done through the supramalleolar osteotomy. Otherwise full correction will uncover the varus subtalar contracture, leaving the foot stuck in varus.
The presence of ankle degenerative changes is important. If the ankle degeneration is significant, then osteotomy surgery may not be indicated. An ankle fusion may need to be considered. To avoid degenerative changes, the proximal fibular migration with ankle valgus should be treated aggressively.
- Limb Length Discrepancy: LLD is a common problem with MHE. Since growth rate is less predictable in MHE, epiphysiodesis is a less reliable option. Furthermore, short stature is a common problem since growth inhibition affects both sides, making epiphysiodesis and its associated height loss, less attractive. LLD is a result of more inhibition on one side than the other. Discrepancy can be due to either or both femur and tibia. The preferred method of lengthening now is with an implantable lengthening device. Since the total LLD is usually 5cm or less, waiting till closer to skeletal maturity is advisable. In cases where the discrepancy is predicted to be much larger, and two lengthenings are anticipated the first lengthening can be around age 8 and the second closer to maturity. Epiphysiodesis is also a consideration for discrepancies under 5cm.
Multiple Hereditary Exostosis (MHE) often involves multiple areas of the upper extremities from the scapulae to the tip of the fingers. Specific areas are of concern may be due to the location consistently causing pain or deformity while others impinge upon nerves and blood vessels. Joint involvement of the wrist and elbow are quite common. The upper extremity has many unique features in MHE in that the function of the forearm and wrist are often severely involved. The radio-capitellar joint is particularly prone to dislocation.
- Scapula: The scapula is a common site for osteochondroma growth. Ostoechondromas of the posterior (dorsal) aspect of the scapula are often not painful but may become quite apparent and can be large and disfiguring. They may be excised for relief of discomfort or cosmetic purposes. Dorsal scapular osteochondromas are divided into supra and infra spinous. The supraspinous tumors can entrap or be near the nerves to supra and infraspinatus. Care must be taken to avoid injury to this nerve and to decompress the nerve at the scapular notch and from around the tumor if necessary. Infraspinous dorsal tumors can be approached directly.Anterior scapula osteochondromas (costal surface) are often painful as they contact the ribs when the patients raise and lower their arms. They also cause the appearance of winging of the scapula when they are large. The approach to the costal side of the scapula for excision is determined by location. More medial ones are approached along the vertebral border of the scapula, cutting through the trapezial fascia and then detaching the rhomboid major and minor and dissecting under the subscapularis muscle to locate and excise the osteochondroma. Care must be taken since the long thoracic nerve can lie on the surface of large protruding osteochondroma as you approach the axillary border of the scapula lying on the Serratus Anterior muscle. Through the vertebral border approach one can reach almost to the axillary border which is where the long thoracic and thoracodorsal nerves are at risk. Lateral scapular osteochondroma can also be excised with a lateral incision. Care to protect the thoracodorsal nerve is paramount. The teres major and minor origins on the scapula may be detached with care not to injure the more laterally lying axillary and thoracodorsal nerve as well as the circumflex artery.The scapula is one bone that is prone to malignant degeneration of tumors. The vast majority of these are low grade chondrosarcomas. These require marginal excision of the tumor but in aggressive and locally recurrent cases partial or complete scapulectomy may be required. One of the problems with scapular tumors is that the constant movement between the chest cavity and the scapula may break off cartilage rests that form free chondroscarcoma loose bodies in the soft tissues between the scapula and the ribs. These can be seen only on MRI.
- Humerus: The proximal humerus is a common location for osteochondroma growth. While many are asymptomatic and can be left in place, medial proximal humeral osteochondromas may cause nerve and arterial and/or venous compression. Ulnar nerve compression is the most common and in excising these tumors the nerves and artery exiting the brachial plexus must be visualized and protected. Excision of large humeral osteochondroma may weaken the shaft of the humerus. Prophylactic fixation of the humerus or excision and grafting may be needed.
- The Forearm: Osteochondromas are particularly problematic when they occur in the forearm. The two bone structure of the forearm makes it prone to problems that do not occur in single bone segments such as the humerus. Osteochondromas growing between the radius and the ulna cause disturbance and limitation of supination and pronation. This is mostly a mechanical problem. This can result from a single osteochondroma extending from the radius or ulna or kissing osteochondromas extending from both bones towards each other. These osteochondromas should be excised early to maintain or restore supination and pronation. Ulnar lesions are frequently sessile budding off of the distal ulna. This creates the appearance of what is referred to as the candlestick ulna. Pedunculated lesions are also common off of both bones. Osteochondromas may also interfere with muscle function and may be excised to allow for untethered muscle excursion.MHE is associated with differential growth of one bone versus the other in the forearm. This is similar to the relatively short fibula compared to the tibia seen in the leg in these patients. Since these two bones normally grow evenly with each other, this differential growth between the two bones often leads to secondary deformities. Ulna bowing is one of these secondary deformities. The ulna may bow into the anatomic radial bow, narrowing the interosseous space. This alters the normal bucket handle mechanics of the radius rotating around the ulna, contributing to loss of supination-pronation. This is often followed by gradual radial head subluxation and dislocation in a similar pattern to acute dislocations seen with Monteggia fracture-dislocations.Treatment for forearm MHE depends on the type. It can be divided into the following 6 surgical options, which are frequently combined:Option 1: Resect osteochondromas
Option 2: Correct ulnar bowing to increase interosseous space and treat or prevent radial head subluxation.
Option 3: Correct distal radial tilt with hemiepiphyseodeis or closing wedge distal radial osteotomy
Option 4: Correct length discrepancy between bones by lengthening or epiphysiodesis
Option 5: Reduce Dislocated Radial head.
This classification aids in treatment and prevention of sequelae of the MHE forearm. A type 1A plus osteochondroma would simply have a resection of the osteochondromas that are limiting motion and impinging on the other bone. While most such osteochondromas are distal, proximal OC pushing out the radial head can also exist.
A type 2b with normal radial tilt would have an ulnar osteotomy for straightening with or without ulna lengthening.
A type 2b with increased radial tit with carpal slip would undergo resection of the osteochondroma, ulna straightening osteotomy and a distal radial closing wedge osteotomy. If there was no carpal slip the distal radius could be watched or hemiepiphysiodesis could be considered. Lengthening of the ulna may be beneficial as well to aid the buttress on the ulna side of the wrist. Getting the ulna out to length is known to improve grip strength.
A type 2c needs to undergo an ulnar lengthening while pulling down the radial head to station. This reconstruction is a salvage procedure and even with open reduction of the radio-capitellar joint and reconstruction of the annular ligament, the patient does not regain the loss of supination and pronation. The advantage of reducing the radial head is mostly to remove the bothersome prominence at the elbow, which is sometimes tender and is prone to be bumped. In some late cases of reduction of the radial head is also performed in conjunction with radial head prosthetic replacement.
The presence of proximal radial neck valgus may be significant in when addressing radial head dislocation. In some cases a varus osteotomy of the proximal radius is performed at the time of open reduction of the radiocapitellar joint.
The goal of treatment is not to allow the radial head to dislocate. This can be achieved by timely treatment of the Type 2b as the ulna bow approaches 15 degrees.
This classification is treatment based and will aid in the decision making tree in MHE forearm deformity.
- Hand and Fingers: Osteochondromas can grow anywhere within the hand and carpus. Common problems include joint impingement by growth of an osteochondroma in the PIP or DIP joints. These often require excision with ligament reconstruction at times. Although resection of finger osteochondromas and osteotomies of the phalanges are not difficult procedures, they are associated with a high risk of loss of motion of the PIP joint that is difficult to recover. Cleinodactly with radial and ulnar deviation of the digits is seen and may require straightening osteotomies if function is impeded. Subungual exostoses are also seen and may cause pain as the nail bed is raised. These are treated by raising the nail and excision of the osteochondroma.
- Spine: Spinal involvement in MHE has been reported as high as sixty eight percent of patients with twenty seven percent of patients having encroachment of the spinal canal. Osteochondomas can grow in any location of the spine but is likely to be an uncommon clinical problem. We still recommend all patients with MHE undergo a spinal MRI by age 8 as undetected spinal encroachment can be clinically dangerous.