The Huckstep Nail for Fixation of Mechanically Deficient Femoral Bone

THE HUCKSTEP NAIL FOR FIXATION
OF MECHANICALLY DEFICIENT
FEMORAL BONE

Nasser M. Kurdy and Charles I. Ayekoloye

The interlocking intramedullary nail remains the gold standard fixation device for femoral shaft fractures in adults.1–3 With improvements in design and locking options, the new generation nails have extended the indications for nailing to include complex and comminuted shaft fractures, subtrochanteric fractures, and combined femoral neck and shaft fractures.⁴ When choosing a fixation device, the presence of mechanical deficiency is an important factor that needs special consideration.

Mechanical deficiency of the femur commonly results from either generalized bone loss due to osteoporosis or from focal bone loss due to metastatic deposits, with both conditions being more prevalent in the older age group. In theory, the use of an intramedullary device in this group of fractures should allow significant uniaxial bending and torsional loads to be carried by the reconstituted bone, which is usually of inadequate quality to allow reconstitution with screws and plates.⁵ However, most of the literature on intramedullary fixation of femoral fractures involves young adults suffering high-energy trauma and does not address age-related comorbid factors or the quality of bone. The literature on outcome of this type of surgery in the elderly population is sparse.^6,7

As life expectancy continues to increase, the incidence of insufficiency femoral neck fractures also continues to increase.⁸ Even though not as common, fractures of the femoral shaft do occur in the elderly population and usually result from low-energy trauma such as a fall from standing height.^6,7 These fractures present a major challenge as in a high proportion of cases, they are associated with significant comminution and can occur anywhere from the subtrochanteric to the supracondylar area of the femur. Morbidity and mortality following femoral fractures in this group of patients are very high and are influenced by coexisting premorbid medical factors in addition to cardiopulmonary complications associated with immobility.^6,7 The fixation device considered should therefore address early weight bearing as well as early mobilization to reduce the complications associated with these fractures in the older age group.

The osteoporotic comminuted femur presents two specific problems when choosing a fixation device. Firstly, osteoporotic bone does not support fixation well as the security of fixation is directly dependent on the bone mineral content.⁹ As such, it is necessary for the fixation device to involve long segments of the bone. Secondly, in the presence of comminution, the construct is inevitably load bearing. Most intramedullary nails in current use have a maximum of two proximal and two distal locking screws, thus risking poor purchase and cutout of the osteoporotic bone.⁶ Therefore, an intramedullary fixation device that distributes the locking mechanism through long segments of the bone would reduce this risk.

Fractures of the femur in the elderly can also be associated with metastatic disease. In this situation, focal bone loss combined with osteoporosis makes stabilization especially difficult. The reduction of mechanical strength of the bone at the site of metastatic involvement is well documented.¹⁰ In the femur, a common location for metastatic deposits is the calcar and subtrochanteric area, where loss of the medial cortex results in an adverse biomechanical environment for extramedullary devices, which regularly fail either from screw cutout or fatigue failure of the implant.⁵ Therefore, in the presence of a mechanically deficient femoral bone, the fixation device must be able to:

1. Achieve early definitive stabilization to allow early mobilization.

2. Provide a strong enough construct that allows early weight bearing.

3. Last the remaining life of the patient or until union of the fracture, without loss of fixation or failure of the implant.

From our reported experience,11 the Huckstep intramedullary compression nail can achieve these goals. This device was designed by Professor R. Huckstep and has been available since 1967. The essential characteristics of the Huckstep nail are strength, rigidity, versatility, and inertness. It combines the mechanical advantage of an intramedullary nail with the rigid fixation of a compression plate, providing a strong fixation construct in difficult mechanical circumstances. It has in fact been referred to as an intramedullary plate.¹²

THE HUCKSTEP NAIL AND INSTRUMENTATION

NAIL DESIGN

The Huckstep nail was designed as a straight solid nail with a rounded square profile and is bullet-tipped. The standard nail is 12.5 mm in diameter, which is the most commonly used size for femoral nailing, and is available in different lengths from 15 to 60 cm. Along its length, the nail has 4.6 mm transverse holes at 15 mm intervals for 4.5 mm fine-threaded locking screws. At its proximal end, the standard reconstruction nail has four oblique holes at 130 degrees that allow 4.5 mm lag screws to be passed into the femoral neck. An alternative reconstruction nail is available with an expanded proximal end of 14 mm diameter, allowing insertion of 6.5 mm lag screws into the femoral neck. There is also an 11.5 mm diameter nail for use in the smaller femur, but it is not strong enough for early weight bearing.

The nail is made of titanium alloy (4% vanadium and 6% aluminum). This alloy has a low modulus of elasticity (about 50% that of cobalt chrome and stainless steel) and is much nearer the modulus of elasticity of bone. The nail has been shown to be 1.1 to 1.8 times stronger than the average femur,¹³ and can therefore permit early weight bearing in most cases. As with most implants, this alloy is inert and the nail seldom requires removal after union.

The square design is thought to lead to less interruption of the endosteal blood supply, and as with most current nail designs, it does not depend on interference fit for fixation.

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