Leg geometry (aesthetics and function)

The legs are indispensable for our mobility. They enable us to stand upright, walk, run, jump, move elegantly or carry heavy loads, even on extremely uneven terrain. In addition, two legs of equal length lead to a horizontal alignment of the pelvis and a vertical alignment of the spine. In order for our legs to be able to fulfil these and other functions, a multitude of prerequisites must be met.

Three important parameters determine the leg geometry

A leg's length is the combination of the length of the thigh length and the lower leg, and the height of the foot. With regard to the alignment of the spine, possible pelvic asymmetries are added. Since the resulting leg length also depends on the angular position between the upper and lower leg, joint angles and joint instabilities also play a role, so that measurements in standing position with correct rotation and under equal loading of both legs are an indispensable prerequisite for the correct recording of leg geometry.

The load axis (load vector) runs from the center of the hip joint to the center of the ankle joint and passes through the center of the knee joint. If the load axis runs to the inside of the center of the knee joint (medial), the deformity is called a bow leg; if it runs outside (lateral), it is called a knock-kneed leg. In both cases, the knee joint is overloaded on one side, especially when body weight is excessive and if the corresponding meniscus is damaged. As a result, the cartilage shrinks, causing the deformity to increase further and the joint will eventually have to be replaced by a prosthesis if no correction of the deformity is made.

While rotation concerns the mobility around the longitudinal axis in a joint, torsion describes the twisting within a bone. As long as the neutral position (zero passage) is reached during normal movement, torsion deviations usually remain unnoticed by the patient. Only when the deviations are so large that the normal neutral position can no longer be reached or when, for example, the range of motion is significantly increased during extreme sporting activity, do constraining forces and consequently shear forces occur in the joints, which lead to evasive movements and thus to premature fatigability. In the case of large torsional deviations, impingement phenomena at the joint boundaries can also become painful and promote joint wear.

Long standing radiographs — the gold standard

The basis for measuring leg geometry are special X-rays taken while standing (Long Standing Radiographs — LSR), in which both legs, the pelvis and also the lower (lumbar) spine are imaged from the front. There are various techniques for taking these images, but we favor the so-called "one-shot technique". With this technique, an X-ray beam is triggered only once, which images both legs, the pelvis and the lower spine simultaneously. Other systems, such as swing-arm systems or scanners, produce images section by section that move along the leg lead to artifacts if the patient moves during the exposure, which is especially common in children. At ZEM-Germany, the first whole-leg detector in Germany made by Fuji was installed at the beginning of 2020. This system delivers excellent image quality with a low radiation dose and much more: a digitally controllable, motorized leg length compensation for exact compensation of leg length differences and integrated scales under both feet enable weight compensation so that both legs are actually equally loaded when the image is triggered. Lateral whole-leg recordings are also possible with this new technology. All LSR measurement parameters are documented and verifiable in the image and are thus available for further planning.

X-ray diagnostics at ZEM-Germany with Fuji detector

Correct planning is what counts

Every corrective operation is preceded by detailed surgical planning, which is carried out on the computer with special software. The reverse planning method (RPM) developed at ZEM-Germany is adapted to distraction nails. The principle is based on the fact that, starting from an "ideal" final result, the individual correction steps are carried out in reverse, taking into account all parameters such as unalterable bone structures, special features of the implants and also the geometric changes in the bone during postoperative lengthening. In difficult cases, different solution variants can be compared in advance to find optimal results. Ultimately, the RPM gives the surgeon a visual guideline to be implemented in the operation theater so that at the end of the treatment the result is as ideal as possible. Compared to external fixators, this procedure is essential because no corrections are possible after the operation, except the lengthening of the distraction nails, and thus the treatment result essentially depends on the correct implementation of the planning during surgery.

There are major differences in distraction nails

Knowing the geometry of the leg, it quickly becomes apparent that simple shortening of the leg without any deformity is rare. There are even joint deformities that cannot be seen at all from the outside. Therefore, it is always necessary to first carry out a complete physical and X-ray analysis, and only then to start surgical planning so that all the above-mentioned parameters can be included in the correction and are correct in the end. Even if there is actually "only" a shortening, the other geometric parameters must not simply be neglected. intramedullary distraction nails lengthen the bone along the anatomical leg axis, which can lead to a lateral shift (knock-knee deformity) of the mechanical leg axis in the thigh (femur). Thus, an initially purely short leg can end up with deformity due to the lengthening, which may then have to be corrected later in a further operation, if not properly planned for from the beginning. This is often overlooked, especially in cosmetic leg lengthening.

Distraction nails are technically complex and require a special surgical technique. There are implants that aim solely at lengthening without taking the leg geometry into account. The FITBONE was designed from the beginning as a multi-directional corrective implant that can be expected to produce accurate results when used correctly.

FITBONE with control unit

The decisive step — the operation

At ZEM-Germany, the surgical plans which have been carefully developed for each patient are transferred directly to the operating theater so that the surgeon has a complete overview of all stages of treatment at his disposal. On a scale of 1:1, the entire leg geometry in the actual condition and size, the immediate postoperative target condition, the ideal final condition at the end of lengthening, as well as all important geometric parameters, are displayed on large monitors. This atmosphere creates the best conditions for the surgeon and maximum safety for the patient, so that the intended treatment result is actually achieved. The ZEM-Germany also has some special features when it comes to performing the operation. In order to minimize the incisions and soft tissue injuries, a particularly gentle surgical technique has been developed. A patented sleeve system (Tube-II system) makes it possible to open the bone precisely where necessary with coordinated surgical portals, to ream a targeted path according to the preoperative surgical planning and to optimally protect both the bone and the soft tissues. In order to ensure optimal orientation during the operation, ZEM-Germany uses an efficient grid plate (double-line grid) with which almost all geometric parameters can be monitored and desired corrections immediately confirmed. This is indispensable especially for the implantation of distraction nails. Due to the exact planning and carefully performed surgical steps, it is thus possible to reduce soft tissue injury to a minimum. The longest skin incision is usually only about 2 cm and heals almost invisibly when running in the skin lines.