Treatment for achondroplasia involves four segment lengthening: simultaneous lengthening of both femurs and tibias in each leg. The major advantage to a four segment lengthening is that it maximizes the amount of length gained while minimizing time in the fixator. This in turn minimizes wheelchair time and minimizes growth inhibition in younger patients.
The lengthening surgery involves an osteotomy (surgical cut to the bone) separating the bone into two segments with application of an external or internal fixator. The fixators will perform the lengthening by slowly distracting the bone segments. As the bone is distracted, new bone will form in the gap. For more information on the biology of limb lengthening, see the Limb Lengthening Center.
There are three different approaches to four segment lengthening:
- External Fixators
- Internal Fixators
The external fixator approach uses external fixators to lengthen each segment (femur and tibia) for each leg. The fixator most commonly used is the Modular Rail System (MRS), although the Taylor Spatial Frame (TSF) may also be used on the tibia to correct any angular deformities. External fixators are used on younger patients whose bone diameters are too small to accommodate internal fixators.
The internal fixator approach uses the PRECICE lengthening nail. We use the PRECICE 2 which was FDA-approved in 2013. The major advantage to the PRECICE is that it removes many of the discomforts associated with external fixators, such as pin site infections, muscle stiffness, and physical awkwardness. One downside to the PRECICE is that all four nails cannot be implanted simultaneously. There is a major risk in reaming the medullary canal of the bone when inserting a femoral PRECICE and a tibial PRECICE at the same time. For this reason, we stagger the insertion of the PRECICE nails by 3 weeks. This necessitates a second surgery. The PRECICE is also too large to insert into the smaller bones of young patients. Currently, it will not accommodate patients 8 years or younger; however, a new device is being developed with younger patients in mind. By the summer of 2015, PRECICE may be available for younger patients.
The hybrid approach utilizes an external fixator on one segment (usually the tibia) and the PRECICE on the other segment (usually the femur). The fixators can all be applied at once in this method since there is no risk to reaming the medullary canal.
During the lengthening surgery Dr. Paley will also correct any limb deformities. Femoral lengthening corrects coxa vara, fixed flexion deformity (FFD) of the hip, and decreases hyperlordosis. In achondroplastic patients the hip does not bend all the way backwards. The fixed flexion deformity of the hip causes them to arch their backs in order to stand up straight, and this position of arched back is called hyperlordosis. This is not a spinal deformity but a hip deformity, specifically caused by the FFD and the body’s natural response to it.
After Dr. Paley performs the osteotomy, he will align the femur and then lengthen it in its proper position. It is important to lengthen parallel to the axis of the ankle, knee, and hip joint, not parallel to the bone. If the fixator is applied parallel to the bone, it will create a knee deformity as the femur lengthens. The upper femoral pins are placed at an angle and the lower pins are placed in line with the bone. When the fixator is applied, this pushes the upper end of the femur backwards during lengthening, straightening out the pelvis. This addresses the FFD and corrects the hyperlordosis. The posture of the hip is maintained due to tethering of the soft tissues. Dr. Paley will also need to lengthen these soft tissues.
Tibial lengthening corrects the bow leg deformities. The Taylor Spatial Frame (TSF) can correct angular deformities while lengthening simultaneously. Bow leg deformity is due to a bony deformity in the tibia and fibular overgrowth relative to the tibia. In patients without bow leg you can draw a straight line from the ankle to the hip and it will pass through the center of the knee; not so when there is a bow leg deformity. The fibula is overgrown in relation to the tibia because the lateral collateral ligament (LCL), which goes from the femur to the top of the tibia is loose, causing the knee to wedge open. By pulling down the fibula, it stretches the LCL and pulls the knee together. Dr. Paley accomplishes this by freeing the fibula ~2 cm near the end of lengthening by removing a screw that fixes the fibula to the tibia. As lengthening continues, the LCL is tightened, pulling the knee into the correct position. This eliminates the varus deformity of the knee, or the “wobbly” knee joint, and corrects the bow leg deformity.
During surgery foot splints are fabricated and applied. The splints are necessary to keep the foot and toes in an upward position throughout lengthening and are required for all tibial lengthening patients. The Velcro straps of the splints are attached to the rings of the tibial fixator.
It is important to prevent spinal stenosis symptoms during lengthening. For all patients, and especially those with spinal stenosis, Dr. Paley avoids this complication by closely monitoring posture. Positioning and moving patients while under anesthesia is done very carefully. All patients receive a reclining wheelchair rather than a straight-back chair. They are not allowed to sit up more then 45 degrees while their knees are straight and their hips are bent. If the knees are bent they can sit up to 90 degrees. This helps to reduce the pressure on the spine. Such small details underscore the vital importance of experience when performing these complex lengthening procedures.
Bilateral humeral lengthening aims to increase reach and improve proportions: hands should reach mid-thigh and arm span should be equal to height. This also corrects the rhizomelic disproportion. More importantly, humeral lengthening significantly improves the patient's ability to maintain their personal hygiene.
For most patients this lengthening is 10 - 12.5 cm (~4-5”). Humeral lengthening is the easiest of the lengthenings. There is very little pain and very little difficulty for the most part. Patients remain mobile in the device and there are very little restrictions. Any elbow deformity can be corrected at this time. Dr. Paley will also allow most humeral lengthening patients to return home for the lengthening, provided they return for follow-up appointments every 4 weeks.
After surgery, the patient will begin the distraction phase (also known as the lengthening phase), usually one to five days after surgery. The time prior to lengthening is called the latency period.
During the distraction, close clinical follow-up is recommended every two weeks. During these visits, x-rays are obtained and the patient is assessed for any developing complications, including pin site infections, nerve complications, premature consolidation, muscle contractures, and regenerate bone fractures. Most complications can be easily corrected when properly identified, thus close clinical follow up is necessary to ensure that complications are spotted early and corrected swiftly. For more information on the potential complications of lengthening and how we solve them, see Treating Complications.
The patient will also be required to attend physical therapy daily, five days per week. Therapy is crucial during the lengthening process, and Dr. Paley will require all patients to stay in West Palm Beach for the duration of the lengthening and receive daily physical therapy at the Paley Orthopedic & Spine Institute Rehabilitation Department.
Once the patient concluded lengthening, they will enter the consolidation phase. The newly regenerated bone is still very weak and the fixator must remain in place until the bone has properly healed.
Dr. Paley typically clears patients to return home during the consolidation phase. He requires that patients send him x-rays on a monthly basis in order to assess bone healing. Near the end of the consolidation phase, the external fixator is dynamized. Dynamization is the process of increasing the weight-bearing taken by the bone and lessening the load taken by the external fixator. This helps strengthen and thicken the bone in preparation for removal of the external fixator.
At the end of the distraction phase and before all adjustments are stopped, it is very important to ensure that the bones are straight. Measurements of joint orientation angles on the x-rays help detect any deviation. If present, the external fixator is adjusted to correct the angles to a normal alignment. Failure to correct this will result in a malaligned bone at the end of lengthening. Dr. Paley will routinely check joint orientation during follow up visits.
Another important adjustment that is required at this stage is derotation. An achondroplastic tibia is often internally rotated. Derotation can be facilitated by the Taylor Spatial Frame. It is for this reason that Dr. Paley prefers to use a circular external fixator (i.e. TSF) for tibial lengthening. The modular rail system is preferred for femoral lengthening since derotation of the femur is much rarer and a modular fixator is more comfortable for the patient at the hip.
Once the bone has sufficiently healed, the patient will return for fixator removal, a minor outpatient procedure. In some cases a small diameter rod is inserted into the bone at the time of fixator removal to prevent fracture.