TFN-ADVANCED Proximal Femoral Nailing System (TFNA)

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TFN-ADVANCED Proximal Femoral Nailing System (TFNA)

Michael Blauth, Christopher Finkemeier, Hiroaki Minehara, Paulo Barbosa

TFN-ADVANCED

Although PFNA and TFN nailing systems have been successfully used in the past, several clinical issues for improvement have been identified by surgeons and engineers. Many of these issues have now been addressed and solved by implant and instrument design changes incorporated into the new TFNA nailing system.

The new nail system comprises short nails (lengths 170 mm, 200 mm, 235 mm) with distal nail diameters of 9, 10, 11 and 12 mm as well as long nails (lengths 260 to 480 mm in 20 mm increments) with distal nail diameters of 9, 10, 11, 12 and 14 mm. Such choice should address a broad range of patient anatomy. All nails are available in Caput-Collum-Diaphyseal (CCD) angles of 125°, 130° and 135°. The long nail provides three distal locking options including a unique oblique distal hole that has an offset angle of 10° to more appropriately target stronger bone in the condyles. Multi-planar locking also offers increased stability.

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Features to avoid anterior cortical impingement

The complications of penetration or anterior cortical impingement while using long intramedullary nails for pertrochanteric femur fractures are due to a mismatch of the femoral antecurvation with the radius of curvature (ROC) of currently available cephalomedullary nails. Bazylewicz et al [1] reported that most of the intramedullary nails with a ROC of 1800 mm ended up in the anterior half of the space available for the nail with 16% within 3 mm of the anterior cortex. Patients that are shorter and/or have an increased femoral bow as measured on a lateral x-ray are more likely to have an anterior nail tip position or cortical impingement [2]. To thoroughly investigate this issue, a comprehensive 3D computer graphical anatomy study of the femur was conducted to serve as a basis for a new nail design [3]. Analyzing 27 Caucasian and 13 Japanese subjects, the ROC resulted in 962±157 mm (Caucasian subjects) and 790±151 mm (Japanese subjects). These results indicate significant differences between ethnicities and that the ROC should be closer to these values instead of 1500 mm, which is a frequently chosen radius in current nail systems on the market.

The new TFNA has a radius of curvature of 1000 mm to improve the anatomical fit and to help avoid impingement of the anterior cortex (Fig 1).

References
1) Bazylewicz DB, Egol KA, Koval KJ. Cortical encroachment after cephalomedullary nailing of the proximal femur: evaluation of a more anatomic radius of curvature. J Orthop Trauma. 2013 Jun; 27(6):303-307.
2) Roberts JW, Libet LA, Wolinsky PR. Who is in danger? Impingement and penetration of the anterior cortex of the distal femur during intramedullary nailing of proximal femur fractures: preoperatively measurable risk factors. J Trauma Acute Care Surg. 2012 Jul; 73(1):249-254.
3) Schmutz B, Kmiec S, Wullschleger M, et al. 3D computer graphical anatomy study of the femur: a basis for a new nail design. 2nd AOTrauma Asia Pacific Scientific Congress & TK Experts Symposium. May 2014; Seoul.

Fig 1 The TFNA nail, illustrated in green, has a 1000 mm ROC and perfectly follows the antecurvation of most femurs. The blue nail demonstrates a less favorable fit of a simulated nail with 1500 mm ROC.

Features to avoid loss of closed reduction during nail insertion

Surgeons often report some loss of reduction during nail insertion, specifically in cases involving nail insertion through a fractured greater trochanter. This often leads to an unintended varisation of the head-neck fragment (HNF) and a medialization of the HNF resulting in reduced bone contact in the calcar area (Fig 2 and 3).

The combination of a large proximal nail diameter and a very lateral entry point has been identified as a potential reason for such a loss of reduction. As a result, a smaller diameter nail with a laterally flattened profile to more appropriately respect the anatomy of the proximal lateral femoral wall would be advantageous. Both design features have been realized with the new nail. The smaller 15.66 mm proximal nail diameter of the TFNA (compared to 16.5 mm and 17 mm for PFNA/PFNA-II and TFN) and the LATERAL RELIEF CUT design (Fig 4) of the proximal nail end serve to reduce the potential impingement of the nail with the lateral femoral wall and the HNF. Both of these issues could result in varus malalignment and a loss of reduction, which remain key indicators for an increased risk of cut-out. The small proximal nail diameter also helps to preserve bone in the insertion area, which is especially beneficial in the femora of small stature patients.

Evaluating nail fatigue is a key stage in the preclinical analysis of new implant designs. The median fatigue limit for the TFNA nail was 24% higher than that of the Gamma 3 nail and 47% higher than that of the InterTAN nail. This increase in fatigue strength is likely attributed to the use of a high-strength Ti-Mo (Ti-15Mo) alloy and the design features of the nail (Fig 4c).

Fig 2a-c A 60-year-old female patient with a 31-A2 fracture (a). Closed reduction on the traction table and insertion of the guide wire (b). With nail insertion, the HNF displaces to medial and varus (c).

Fig 3a-d Closed reduction and nail insertion (a). With advancement of the PFNA, the HNF displaces to medial. This cannot be avoided by pushing the shaft from lateral with a ball spike pusher (b). The attempt to reduce the calcar with a collinear clamp results in pronounced varus malalignment (c and d).

Fig 4a-c The LATERAL RELIEF CUT design of the nail (a) avoids impingement of the lateral cortex (b). The BUMP CUT design of the proximal hole for the head element (c) provides improved fatigue strength compared to existing nails of similar size.

Features to avoid suboptimal placement of the head element

Apart from the newly introduced nail design features, which help to maintain a good reduction, it is also essential to place the head element in the correct position of the femoral head to avoid cut-out or cut-through. Numerous studies have demonstrated that a center/center position of the head element ensures the best clinical outcome. Multiple instrument features, including a multi hole drill sleeve for the facilitation of precise nail entry and aiming aids to accommodate the placement of the head element guide wire in the correct position have been added to the TFNA system to enable accurate implant placement. The insertion handle is radiolucent and has radiographic indicators to help the surgeon with exact guide wire placement for head element positioning in the lateral view (Fig 5). This feature, together with the guide wire aiming device, which checks guide wire position in the AP view, is influential in the placement of the guide wire in the center/center position of the femoral head. It also helps to reduce the number of imaging maneuvers and x-ray shots required.

Fig 5 Illustration of the radiolucent aiming arm (a). Nail rotation has to be adjusted until the two radiographic lines on the insertion handle are parallel to both the femoral shaft and nail. This ensures that the guide wire is in the correct position in the lateral view (b, c). As a prerequisite for this, a true lateral projection of the proximal femur (ie, a 180° angle of the femoral neck and shaft) has to be established by rotating the C-arm from a neutral position to about 15° to compensate for the anteversion of the head and neck.

Features to avoid cut-out and cut-through

Multiple biomechanical and finite element (FE) studies have illustrated that the purchase of implants in osteoporotic bone is compromised. A blade-shaped head element and augmentation have been proven to enhance implant stability and this is especially significant in a society with a growing ageing population and increasing cases of osteoporosis. Having a modular nailing system, which comprises a screw, blade, and augmentation, offers a distinct advantage when addressing specific fracture situations, local bone quality, and issues like suboptimal reduction and implant placement. The surgeon has the option to choose between the TFNA Helical Blade and the TFNA Screw for head element fixation (Fig 6), which accommodates differing surgical preferences and facilitates hospital standardization. It is recommended to use the helical blade in cases of poor bone quality because it allows for bone compaction around the head element and avoids the bone loss that occurs with the drilling and insertion of the standard hip screw. Optional holes in the blade or screw enable augmentation of the head element in cases where additional fixation is required (only in countries where augmentation is approved from a regulatory perspective). The benefit and efficacy of augmentation is of particular significance in an off-center position of the head element.

Fig 6 The TFNA Helical Blade (a) and TFNA Screw (b) have an oblique lateral end that lies flush with the lateral cortex, therefore reducing head element protrusion into the soft tissues. Both the helical blade and screw are available in lengths of 70 to 130 mm with 5 mm increments.

Features to avoid leg shortening and lateral protrusion of the head element

For reasons of versatility, the new TFNA system offers two locking options. The first option locks rotation of the head-neck element. The second option inhibits lateral sliding of the head-neck element, thus preventing shortening of the femoral neck and lateral protrusion of the head element (Fig 7).

Fig 7 A built-in locking mechanism (a) facilitates rotational locking (b), which allows sliding of the screw or blade head element while blocking rotation. Static locking can be achieved by tightening the locking mechanism with a torque limiter to create a fixed construct with no head element movement. The static locking mode maintains the femoral neck length.

Indications

The TFNA system is indicated for:

Stable and unstable pertrochanteric fractures
Intertrochanteric fractures
Basal neck fractures
Combination of pertrochanteric, intertrochanteric, and basal neck fractures

The long nails are additionally indicated for:

Subtrochanteric fractures
Pertrochanteric fractures with shaft fractures
Pathologic fractures (including prophylactic use) in both trochanteric and diaphyseal regions
Long subtrochanteric fractures
Proximal or distal nonunions, malunions, and revisions

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