Küntscher made a difference between a ‘nail’ on one side and a ‘rod’, or a ‘pin’ respectively, on the other [6, 7]. The function of a nail, he said, is based on its press fit within the material into which it is driven by a hammer. He compared the intramedullary nail with the nail of a carpenter which is tightly surrounded by impacted wooden material (Fig. 2.2). In orthopaedics, this fixation principle had already been realized by Smith-Peterson with his lamellar nail driven into the impactable cancellous bone of the proximal femur [8]. Since cortical bone of the diaphysis cannot be impacted like wood or cancellous bone, Küntscher concluded that the compressive forces by which the aspired press fit could be achieved, should come from the nail itself. A nail with a combination of strength and elasticity was created for this goal: strength to hold a broken long bone in anatomic axial line even under conditions of functional strain with high bending forces; elasticity to allow for elastic deformation of the nail in firm contact with the hard bone of the surrounding cortex. By what he called ‘transverse elastic deadlock’, Küntscher intended to apply longitudinal stability and torsional stiffness. He got what he wanted from modern development of the German steel industries. In 1920, they had found a steel alloy capable to withstand high strains even when forged to very thin sheets, named V2A [9]. These basic characteristics of the alloy were completed by being stainless which made the material most attractive for its use in the human body.

The first generation of intramedullary nails Küntscher had used in clinical practice had a triangular shape in cross section (Fig. 2.3a). These were easy to construct. But the area of contact between nail and cortex was limited to the lines along the three edges of the implant thus producing the highest strains along these lines. Nowadays, based on our experience, we can imagine that, due to reactive bone resorption, these nails tended to loosen under high repetitive strains and under unfavorable biomechanical conditions.

With new production techniques, it became possible to construct a much more complex implant with a clover-leaf shape in cross-section and a dorsal slot. The medullary cavity was prepared by reaming. The diameter of the nail had to be slightly bigger than the diameter of the last reamer. Hammered into the medullary cavity, such a nail is exposed to circular compression by the cortex. Elastic deformation of the nail reduces its outer diameter and the width of the slot (Fig. 2.3b).

Küntscher’s concept of ‘transverse elastic deadlock’ has widely been refuted by experimental findings. After tight insertion of slotted femoral nails, cutting both nail and femur into segments of 2 cm length revealed hardly any segment with transverse compression of the nail [10]. Based on the clinical observation of general slight deviations of the medullary cavity from a theoretical straight central axis of the femoral diaphysis and from the central axis of the nail, the concept of ‘longitudinal elastic jamming’ of the nail inside the medullary cavity was enunciated (Fig. 2.4). According to this concept, eventual transversal compression of the nail and minimal constriction of the slot are just secondary effects of longitudinal deformation of the nail [11].

Compared to conventional plating or external fixation, Küntscher nailing provides, by far, much less torsional stability to the fractured bone [12]. In simple and stable shaft fracture types, control of fragmental rotation is given by interdentation and cortical contact. In more comminuted fracture types, however, the risk of secondary loss of reduction in terms of length and rotation became obvious in clinical practice. Already in the early 50s, attempts to control secondary malrotation and loss of length were made by adding locking devices (wires and screws) to nails which were changed in shape and design [13, 14]. The development of interlocked nailing started with slotted nails. 1952 Herzog (Krefeld/Germany) published the use of a slotted femoral nail but turned the slot from dorsal to lateral. On the medial side, the nail was perforated by a series of holes. This allowed for ‘interlocking’ with Kirschner wires inserted from lateral to medial through the slot (Fig. 2.5) [13].

It took another 16 years until Küntscher himself proposed to exclude contractive tension of the muscles from intramedullary fixation by means of transverse proximal and distal screws through both bone and nail for comminuted fractures exclusively. The systematic use of interlocking nailing started with the development of static and dynamic interlocking nailing (Fig. 2.6) by Klemm and Schellmann (Frankfurt/Germany) in 1972 [15]. They used conventional slotted Küntscher nails with the only modification of proximal and distal double medio-lateral holes for interlocking screws. They clearly differentiated between static and dynamic interlocking nailing. According to this concept, stable fracture types are fixed with dynamic interlocking performed only at either the proximal or the distal end of the nail but always on the long fragment. For stable midshaft fractures, the nail is used without interlocking screws. Twelve years later, in 1984, Winquist et al. underlined the validity of this concept when they presented their experience with 520 cases of femoral fractures all treated with intramedullary nailing together with their classification of (diaphyseal) comminution [16].

In order to enhance callus maturation of unstable fractures, secondary dynamization was advocated when callus was visible on radiographs, but only on the long fragment. Today, we perform secondary dynamization only if it is clearly indicated in order to avoid delayed fracture healing or nonunion.

Grosse and Kempf (Illkirch/France) designed femoral and tibial nails with a proximal unslotted segment (Howmedica®) [17, 18]. The objective of this change was to allow for a firm connection of a standard aiming jig for proximal guided interlocking with nails of different sizes and the use of an antero-posterior locking screw at the proximal end of the tibial nail. The first interlocking nail of the AO, launched in 1988, i.e. 16 years after the start of the method in Frankfurt, was a slotted nail, too. Besides some changes in bending of femoral and tibial nails, Synthes and the AO community introduced the concept of the ‘dynamic’ locking hole at the proximal end of the nail. The locking screw in a long hole is meant to allow for longitudinal dynamization and, at the same time, to prohibit postoperative rotational displacement of the fracture. Two main objections against this concept of a proximal ‘dynamic’ hole have to be presented. First: Küntscher has clearly explained what has been proven in every day’s clinical practice of intramedullary nailing over decades, that stable fractures under dynamic load achieve torsional stability by interdentation of the fragments (Fig. 2.7). Küntscher used the term ‘Hirth interdentation’ taken from the mechanics of a crankshaft in early car production [6]. In 18 years of primary dynamic interlocking nailing according to the concept of Klemm and Schellmann, not even one case of secondary rotatory displacement of a stable fracture (transverse or short oblique) has been described. Second: Primary dynamic fixation or, if needed, secondary dynamization is not only applied to infra-isthmal fractures of tibia and femur, but also to supra-isthmal fractures. The latter demands distal dynamic conditions which, in the logic of a ‘dynamic hole’, would mean a locking screw in a long hole at the distal end of the nail. A long proximal ‘dynamic’ hole is indispensable for compression nailing but that is a different story (see Sect. 2.8).

Slotted nails have proven to be very unstable in terms of intrinsic torsional stiffness [19, 20]. In addition, the thinner the wall of the nail, the bigger the difference between resistance to bending in the frontal plane (if the slot is located dorsally or ventrally) and resistance to bending in the sagittal plane [12]. This explains why, under strong bending load due to major difference in curvature between nail and medullary cavity, slotted nails get distorted during insertion [21]. In some cases of femoral fractures, torsional deformation of the slotted nail up to 90° has been observed, thus changing the direction of interlocking from lateral to medial towards anterior to posterior [11].

With further evolution of interlocking nailing, the use of slotted intramedullary nails has widely been abandoned. Transverse elastic deadlock has completely lost its importance. Already at the start of interlocking nailing in the 70s, a nail diameter of 0.5 or 1.0 mm less than the diameter of the last reamer used was chosen by the surgeons. Instead of under-reaming, the medullary space for the nail is over-reamed now.

The idea of unreamed nailing initiated the development of thinner nails, thin enough to be inserted all along the medullary cavity without any preparation and strong enough to withstand the usual strains of intramedullary nails. At first, hollow unslotted stainless steel nails with triangular shape in cross-section (Delta nail) were used in clinical practice. Then solid stainless steel nails for femur and tibia (UFN® and UTN®) were brought to the market. Primarily developed as temporary internal fixators for open fractures, they were taken as standard intramedullary fixation devices for all kinds of diaphyseal fractures. Finally, the first generation of intramedullary titanium interlocking nails was introduced by ACE, a Californian company. Titanium nails are more elastic than stainless steel nails and closer to the mechanical properties of intact diaphyseal bone. They seem to have a beneficial effect on fracture healing, strength and mineralization after 12 weeks [22].

Titanium in any version of alloy produces a layer of black oxides on the surface and releases them to the surrounding tissues together with toxic ions of aluminium, vanadium or niobium. ACE has started a technique of surface treatment for titanium trauma implants which was taken from airspace technologies, called ‘tiodizing’, an electrolytic process which enhances anti-galling and mechanical resistance without any dimensional change.

Nowadays, surface modification by anodizing is applied to all titanium implants in two alternative versions with different effects on mechanical and surface properties [23]. The first is type II anodizing corresponding to the ‘tiodizing’ process of ACE, which changes the surface of the implant up to 5 μm (0.000 005 m) in depth. The surface is more bio-compatible and more (44 %) resistant to corrosion. It diminishes the adhesion of osteocytes by reduced protein absorption and diminishes inflammatory or allergic reaction. It prohibits the release of toxic ions.

The second is the ‘type III anodization’ (colour anodization) by which a layer of 20–200 nm (0.000 000 2 m) thickness is built up consisting of colored titanium oxides. Depending of the thickness, the implant surface gets a modification of a specific shiny color, either red, yellow, blue, green etc. Color coding of implants simplifies dramatically the clinical use of complex implant systems. In contrast to the previous type, this type of anodization enhanced osseointegration [23].

Transiently, severe concerns about the destructive effects of intramedullary reaming reached such a level that, during the 90s, unreamed nailing got a status of a dogma, especially in the AO community of Germany. During this short period, the negative effects of intramedullary reaming have completely been exaggerated based on misinterpreted experimental [24, 25] and clinical findings. These data supporting the dogma of unreamed intramedullary nailing were all collected with the use of the former Synthes reamer system which, submitted to meticulous investigation itself, turned out to be the true source of a bundle of increased troublesome reaming effects like cortical necrosis, dead bone sequestration, delay in callus formation and higher rates of infection. In other centers using reamers with deeper flutes, experimental [26, 27] and clinical research [28–30] revealed full thickness survival of the cortex under periosteal blood supply, better periosteal callus production and faster healing of diaphyseal fractures after reamed nailing, compared to unreamed nailing and corroborated the original findings of Gerhard Küntscher.

Unreamed nailing presumed the use of thinner nails with mechanical qualities equivalent to those of conventional nails. New small diameter nails with modified cross-sectional shape or made of new materials appeared on the clinical stage. Each of these aimed at a compromise between necessary strength, elasticity and fatigue resistance. The new material beside the latest developments of stainless steel was a new titanium alloy (Ti6Al4V or Ti6Al7Nb), offering mechanical qualities being closer to those of natural cortical bone than stainless steel. Reduction of the nail diameters induced a higher failure risk of either nail or screw.

Nowadays, reaming of the cavity is done much less aggressively. Cortical reaming is generally avoided (“ream to fit”) and a nail with a diameter of 0.5 or 1 mm less than that of the last reamer head is chosen. No slot and no circular compression are needed any more. Nails are much thinner now than in former times. Of course, these nails do not provide fixation stability by themselves but by the interlocking construct at both ends. The physical characteristics of the locking site are decisive for the amount of fixation stability gained by intramedullary fixation.

Changes of nail design did not come at once after introduction of interlocking screws or bolts. They came with little steps, based on the expansion of our knowledge and experience. The design changed from thick, slotted cloverleaf shaped, stainless steel nail types of Gerhard Küntscher to thinner, solid or cannulated, unslotted nails made of new material like better stainless steel, new titanium alloys or even modern polymers enforced by carbon fibers. Despite these changes, we still talk about ‘nails’. Küntscher would have called all our modern intramedullary implants ‘rods’ or ‘splints’ because they are lacking circular press fit after insertion.

There is some clinical evidence that Küntscher’s concept of ‘transverse elastic deadlock’ was functioning nevertheless. Those who have collected experience with the removal of Küntscher nails after longer periods of time will certainly remember cases in which complete extraction of a tibial nail was impossible even with most sophisticated tricks, extra instrumentation and progressive destruction of the exposed upper part of the nail (Fig. 2.8). In these rare cases, the distal part of the nail could not pass the narrow isthmal zone. Effective transverse elastic compression of the distal part of the nail (closure of the slot) would have been necessary. This, however, was inhibited by bone ingrowth into the wider infra-isthmal slot (Fig. 2.9). This is a proof of closure of the slot at the isthmus and elastic release and widening of the slot below the isthmal zone.