Orthopedic Teaching Cases 2023, Jan, 20 | 12:00 am

NAILS AND EXTERNAL FIXATORS

 

  1. How do IM nails and external fixators differ with regard to load bearing and their moment arm?

 This depends on the fracture type. For AO type A and B fractures with cortical contact, the IM nail or external fixator is load sharing as there is some cortical contact between the main fracture fragments. In a type C fracture, the nail or external fixator is load bearing.

  Although a plate has a short moment arm when fixed to bone, an IM nail is placed down the centre of the intramedullary canal, such that the anatomical axis of the bone is collinear to the long axis of the IM nail. This results in a negligible moment arm and therefore increased stability. An external fixator has the longest moment arm of all three devices given its distance away from the bone.

  1. What different designs of IM nail do you know and what effect do these differences have on performance and stiffness?

 IM nails come in many different designs. They can be cannulated or solid, slotted or non-slotted, cylindrical or clover leaf-shaped. Cannulated nails are less stiff than solid nails, but can be inserted over a guide wire and allow for deformation during insertion, which makes them less likely to lead to a blow out. Also, micro-motion at the fracture site will encourage secondary/indirect bone healing, which is how fractures heal when treated with IM nails.

  Slotted nails decrease the torsional stiffness as well as bending stiffness and are rarely used these days. Clover leaf-shaped nails are of increased stiffness compared to cylindrical nails.

  1. What is the working length of an intramedullary nail and what is the importance of working length?

 The working length is the length of nail between the most distal point of fixation in the proximal fragment and the most proximal point of fixation in the distal fragment. In other words, it is the unsupported portion of the nail between the bone fragments.

  Torsional stiffness is inversely proportional to working length. Bending stiffness is inversely proportional to the working length to the power of 2.

  1. How does the radius of the nail affect rigidity?

 For a solid nail, the rigidity/stiffness (to both bending and torsion) is proportional to the radius to the power of 4. For a cannulated nail, rigidity/stiffness is proportional to the radius to the power of 3.

  1. What is the difference between a reamed and an unreamed nail?

      There are many differences between reamed and unreamed nails.

  Biomechanically, reaming can have several theoretical advantages. It can decrease the working length of the nail in addition to allowing for the insertion of a larger diameter nail. The result of both of these is an increase in the rigidity/stiffness of the construct.

  Unreamed nails theoretically minimise disruption of the endosteal blood supply. However, with reamed nails, there is a six-fold increase in the periosteal blood supply and the direction of the blood flow can reverse from centrifugal to centripetal. Furthermore, there is reconstitution of the endosteal blood supply after 6 weeks. More importantly, the clinical relevance of reaming is such that reamed nails have been shown to have a positive effect on bone union in terms of both rates of union and time to union. This is particularly true in femoral fractures (both open and closed) and closed tibial fractures. The benefits of reaming appear to be less in open fractures of the tibia in clinical trials.

  Finally, there is no good evidence to suggest an increased rate of compartment syndrome or pulmonary complications, for example, fat emboli (which occurs with both techniques), when using a reamed nail over an unreamed nail.

  1. What are the indications for an external fixator?
    1. – Temporary fixation
      • Damage control orthopaedics
      • Periarticular fractures: ‘Span – scan – plan’
      • Pelvic ring injuries (rarely used these days due to pelvic binders)
    2. – Definitive treatment
      • Significant soft tissue injuries with associated fractures
      • Paediatric injuries (since they heal faster than adult injuries and the frame is less bothersome to a child)
    3. – Reconstruction
      • Deformity
      • Infection
      • Non-union
      • Lengthening
  2. How does one increase the stiffness of an external fixator?

 The most important aspect to an external fixator for increasing stiffness (other than good cortical opposition at the fracture site) is the diameter of pins. The bending stiffness of pins is proportional to the radius of the pins to the power of 4. However, as a rule, no pin should be greater than one-third of the diameter of the bone due to the risk of fracture. Other techniques to increase stiffness include:

      • Pins in different planes (circular > multiplanar > uniplanar)
      • Increasing the number of pins
      • Increasing the space between pins
      • Placement of pins near fracture site
      • Decreasing the distance of rods from bone (this reduces the moment arm)
      • Increasing the diameter of rods
      • Increasing the number of rods