Internal Fixation

Internal Fixation Part 2 - Fracture Biomechanics

A fracture heals biologically. But biology is governed by mechanics. Bone cells are mechanosensitive. They respond to motion, strain, compression, shear, and tension. And that means one central truth: The surgeon is not simply fixing bone. The surgeon is engineering the conditions under which biology can succeed. Strain in terms of bone healing describes the amount of motion relative to the fracture gap. Too much strain, and the local biology will favor granulation tissue formation. Small amounts of strain will lead to cartilage or callous formation, and very little strain sets up the environment for bone formation. The surgeon is tasked with determining the fracture needs - sometimes absolute stability (low strain) is required and sometimes relative stability. For example, in deformity correction and reconstructive surgery, the desire is to create absolute stability. In certain fracture scenarios, such as comminution, relative stability is preferred to allow the multiple fragments to unite without excessive compression and without further devitalizing bone. The choice of fixation- the construct, the materials, and the placement - are all variables a surgeon puts together in the operating room depending on the specific circumstances.  The content of this podcast is for educational and informational purposes only and does not constitute medical advice. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.

Internal Fixation

Understanding internal fixation is critical to our later discussions on surgical techniques for a variety of conditions. We started in Episode 1 of Season 2 discussing bunion surgery. This was meant to be an introduction to how we approach realignment and repair of bone segments. Everything we have learned about fracture fixation in emergent conditions, we have refined and honed to use in our reconstructive efforts. Bunion surgery involves just about every surgical principle we will cover in this season - soft tissue envelopes, anatomically safe corridors, biomechanical tension and compression, deformity realignment, and bone fixation.  Throughout human history we have suffered broken bones. Only until recently were broken bones stabilized with devices more complicated than a tree branch and leather straps. Once we had a grasp on aseptic techniques, metallurgy, and the biologic processes involved in bone healing, modern internal fixation of bone injury was achievable.  The current methods and devices are essentially derivatives from the Swiss AO group, a collection of surgeons who outlined the important principles for the use of these devices: anatomic reduction, stable fixation, preservation of blood supply, and early mobilization of joints.  Manufacturing has come a long way, with forging and machining of basic screws and plates being replaced with 3D printing and patient-specific prostheses generated from complex weight bearing CT scans.    The content of this podcast is for educational and informational purposes only and does not constitute medical advice. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.

Bone Healing

Bone is a Living Tissue. It is formed of living cells in a matrix designed to withstand load and to constantly replace and repair itself. When injured, bone immediately begins the repair process, with dedicated cells immediately taking over and beginning a complex signaling event that ultimately results in complete restoration.  Because healing is phase oriented, certain systems must in place at the right time to effect the healing cascade. This begins with inflammation, which is immune system mediated. A hematoma forms, which is filled with platelets, cytokines and growth factors. There are important meditators at work- PDGF (platelet derived growth factor), VEGF (vascular endothelial growth factor), BMPs (bone morphologic proteins), interleukins, TGFa (transforming growth factor), and more. These signaling molecules recruit osteo-progenitor cells and ignite angiogenesis. The system must maintain balance between normal inflammatory response and excessive or prolonged inflammation which can impair the process.  Angiogenesis means "new blood vessel formation," a process critical to repair. The biology of bone healing thus begins with restoration of blood flow.  There are two main types of bone healing - primary and secondary. Primary healing is direct laying down of matrix between two fracture ends. This occurs when there is adequate blood flow, close approximation of bone ends, stability at the fracture site, and low tissue strain. Secondary bone healing occurs when there is relative stability, not absolute rigidity. This involves a cartilage intermediary, a tissue type visible in the healing process.  This concept of strain is critical to understanding how bone behaves. Strain is basically the change in gap length divided by the original gap length. Different tissue types tolerate different amounts of strain. Granulation tissue tolerates high strain, bone tolerates very low strain. This means that the tissue type that will form at a fracture site is heavily dependent on the mechanical environment. Bone healing does not like extremes. Too much motion leads to delayed healing and persistent fibrous tissue. Too little will suppress callous formation and stress shield the bone leading to poor biologic recruitment.  Healing likes the environment to be stable enough but not necessarily absolutely rigid. This is the art in surgical practice - determining how and when bone needs stability, how much stability, and when to progress to load and stress again.  Of course, mechanics alone are not a sure bet that the bone will heal. Biologic constitution also is a factor. Low vitamin D, low calcium levels, high sugar levels, presence of carbon monoxide, and more can lead to poor healing of bone tissue.   The content of this podcast is for educational and informational purposes only and does not constitute medical advice. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.

Bunion Surgery

There is no simple solution to a complex problem. This is true in life, and in bunion surgery.  A bunion is a complex condition resulting from either structural anomaly or biomechanical instability, and often both. Careful evaluation of the condition is required; no two bunion deformities are identical. Mistakes are made when patient expectations are unrealistic, physician capabilities are limited, there is no sound surgical plan to address every component of the deformity, or the procedure is poorly executed. The failure often occurs before the procedure is carried out. Proper planning includes complete evaluation for causation. Removing the bump may provide short term relief, but not addressing all the causative factors will result in long term failure. That is why clinical and radiographic parameters are so critical. Along with an arsenal of procedure selection in the surgeon's pocket to choose from.  Distal osteotomy procedures are the least technically demanding and have the lowest complication rates. The good news, most patients will fall into this category. More proximal procedures are required for extremes of the deformity - rigidity, age of patient and deformity, and profound dynamic forces acting across the first ray. For the segment of patients that fall into this category, expectations should be set accordingly - the more complex the procedure, the more can go wrong. Nonunions, neuritis, hardware complications, and load transfer related issues all have a higher incidence with these procedures.  The content of this podcast is for educational and informational purposes only and does not constitute medical advice. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.

Biomechanics

The bones of the foot are arranged to not only provide stability and absorb impact, but to provide maximal efficiency in movement. Beginning with their axes of motion, which are purposefully arranged to allow motion in a specific sequence as the body's weight transfers over the foot from posterior to anterior. The ankle allows dorsiflexion, the subtlalar and mid tarsal joints allow pronation (shock absorption), which  converts to supination (rigid lever), returning motion back to the ankle as plantar flexion. It is a spring-loaded structure. And stability is not just structural - it's dynamic.  It is crucial to understand that the foot and ankle are merely a system of levers and pulleys. Understanding the anatomic arrangement of the tendons around an axis describes the relative contribution of a muscle (by way of tendons) to the system. The larger the tendon, the more force it can exert. And the farther from the joint axis, the greater the lever arm, also resulting in more force. And when a tendon crosses over a joint axis, we can determine which direction the force will occur.  Tendons on the lateral side of the ankle will pull the foot into eversion, while tendons on the medial side will invert. Posterior tendons plantar flex, while anterior tendons dorsiflex. However the arrangement becomes more complex when we see tendons crossing multiple axes, and becoming maximally loaded and thus effective at specific points during the gait cycle. Pathology occurs when joint axes shift from ideal, or motion is somehow limited either by physical restriction (tight tendons or boney impingement) or by anatomic malalignment. Such is the example with a short or elevated first ray, or a first ray with an axis moving away from the traverse plane. The result of inefficiency in the peroneal complex to sufficiently load the medial column will result in prolonged pronation, the downstream effects being hyper mobility, poor shock absorption, distal deformity such as bunions and hammertoes, and load transfer issues like metatarsalgia, stress fractures, and neuromas.  Thus you can appreciate how biomechanics is the foundation of many of the pathologies we see in foot and ankle medicine. Correcting these deficiencies is essential to surgical practice.    The content of this podcast is for educational and informational purposes only and does not constitute medical advice. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.