53
3. The development of the Pinless external fixator: from the idea to the implant
R Frigg AO/ASIF Development Institute Clavadelerstr., CH-7270 Davos This paper describes the individual stages of development which led to the realisation of the Pinless clamp.
3.1.2 Have Pi
3.1h~uction 3.1.1 Req-ts The development group was given the task of developing a system which would permit fixation of a fracture of a long bone without penetrating the medullary cavity. The following requirements were used as guidelines ing the entire development phase: 1.
Simplicity The system foolproof.
To comply with all these requirements, it was apparent that an external fiiation system was called for. Any internal fixation procedure requires extensive surgery and would be inappropriate as a temporary fixation method. The advantages of the new Pinless system should lie in its simplicity and minimal additional trauma to the bone and soft tissue.
should
be
logically
constructed
systems been used before ?
An examination of existing patents showed that various “pin-less” systems had already been developed. All these systems held the bone between at least two metal points, the exception being Malgaigne’s “point” in which one side is held firmly in a strap.
dur-
and
2.
Rapid application Since the new system was to be designed as a temrxcrn, surgical procedure should be kept to
3.
Minimal damage to soft tissue and bone After temporary fixation of the bone using this system, the surgeon must still have every possible internal fixation proced~ open to him for the definitive stabilisation of the fracture.
4.
It should be possible for an inexperienced surgeon to apply the new system In disaster areas, war zones or less well-equipped hospitals, it is essential that a fracture be carefully immobilised in order to transport the patient to a place where extensive medical care is available.
5.
The number of instruments (and implants) should be kept to a minimLun The more instruments and implants used in a system, the more complex it becomes and the greater the number of possible errors. The system becomes confusing and instruction more difficult.
Fig. 1: J.F.Malgaigne’s
“point! (1840).
This development was followed by the Malgaigne which was used for fixation of patellar fractures.
Fig. 2: J.F.Malgaigne’s
“claw” (1643).
claw
s4
All the other pinless devices described had forfeited their simplicity on account of a complex, mechanical apparatus. When these systems were being developed, they were intended as final fixators for the stabilisation of fractures and in this sense their complexity is not surprising. The requirements were not the same at that time as they are now.
of the clamp would have to be quite considerable if it had to accommodate the soft tissue. Such a wide extension of the clamps led to even greater instability. Initially, the C-clamp with an adjustable tip on one side seemed very promising. The advantage of this clamp was that the function of the clamp could be actively adapted by the surgeon. The C-shaped arms could be made stiffer, i.e. with a minimum of elasticity. Once the clamp tip connected to the fixator arm was positioned on the bone, the adjustable tip could be pressed into the bone using an instrument. A special washer prevented slippage. This meant that the adjustable tip could subsequently only be moved in one direction.
Fig. 5: Two sizes of clamp with adjustable Fig. 3: J.H. Nicholson
tip.
et al. (1927). Femoral splint.
These existing internal “pin-less” fixation systems have no place in modem internal fixation since they contravene all the laws of biological internal fixation. Even if they only caused minimal damage to the bone, they would still destroy the soft tissue covering and thus the blood supply.
However, loading the clamp on one side only had the disadvantage that the tip of the fixator arm did not gain sufficient purchase on the cortex. The greater part of the force applied to the adjustable tip was taken up by the elasticity of the fixator arm, thus preventing the fixed tip from sinking into the far cortex. An attempt to hammer the tips into the bone failed because the opposite tip then sprang loose. In order to reduce the elasticity of the fiiator arm temporarily, special pincers were designed to hold the arm and transfer the force directly to the tips. This tension mechanism failed because once the pincers were removed the fiiator arm was no longer taut and the tips were inadequately anchored in the bone.
Fig. 4: Robert L. Judet, (1979).
3.2 Constructionphase: flexibleclamps
Fig. 6: Fixator arm pincer.
With the above-mentioned, unsuitable designs in mind, we began to develop a Pinless system which would meet modem demands. Initial attempts were made using a Cshaped spring similar to that suggested in the early 80’s by B.G.Weber for temporary, intraoperative bone fixation. However, it soon became apparent that the clamping force of the C-shaped spring was inadequate and that the size
3.3
Develapmentphase: rigid clamps
Somewhat disappointed, the idea of a preloaded, springlike clamp was abandoned and work on construction of a rigid clamp commenced. The fiiator arm was constructed in the form of an I-beam-support. The clamps were
s5
Frigg: Development of the Pinks system
anchored in the bone by rotation of two adjustable tips. The bone fixation achieved was excellent. In addition to the clamping power achieved by rotation of the trocar points, the anchorage of the tips in the cortex was improved because the trocar points could drill into the bone.
sites of a lag screw. The elastic nature of the forceps allows the surgeon to feel whether the tips are slowly slipping, sinking into the bone, or pushing the fracture fragments apart
Fig. 9: Pointed reduction forceps Fig. 7: Rigid clamp. The price of good anchorage in the bone was the increased difficulty in mounting the clamp. Although it was possible, after some practice, to mount the clamp under direct vision, without it, it was extremely difficult. Feedback from the clamp to the surgeon’s hand was poor making it impossible for him to tell whether the tips were well positioned on the bone or not. This meant that it tended to slip off. Reservations about the consequences of bone resorption around the tips during the course of treatment leading to a loosening of a rigid clamp with no elastic preload, led us to abandon this type of fixation.
3.4 Development phase: The forcepa principle
To use the forceps, the bone is fit properties described above are even more the clamp is to be mounted on the bone the soft tissue). Tests using large and forceps confirmed that the problem of solved.
exposed. The indispensable if blind (through small reduction the clamp was
3.5 Devehpment phase: Elaboraiion of the forcepa
pinciple The main task of further development was now to simplify the reduction forceps and integrate them into an external fixation system. Existing reduction forceps were altered so that the handles could be removed. This only led to a partial reduction in volume because part of the handles had to be left in place in order to accommodate the catch.
The investigations carried out with the rigid clamp demonstrated that the success of the Pinless external fiiator did not depend on the rigidity of the fixation but rather on the simplicity of its application. In other words the surgeon must be able to feel whether the clamp is properly positioned or not. A clamp should match the extended index finger and thumb of the surgeon, since these digits form the best “clamp”.
Fig. 10: Pointed reduction forceps with removable handles. To reduce the volume of the forceps down to an acceptable size, the locking catch and the hinge had to be redesigned. A special hinge was made which could be locked in any position using a nut or screw. The arms of the forceps were pressed into holes in the hinge and fixed in place. Opposite these holes, two further holes were made into which the handles could be inserted. Fig. 8: The best “clamp”. A temporary fixation is usually simply and successfully performed using reduction forceps. By feeling the tips of the forceps are placed at the optimal insertion and exit
S6
Fig. 13: Pinless clamp with detached strut
Fig. 11: The first Pinless clamp. Having mounted the Pinless clamp, easily be completely removed.
the handles
could
Fig. 14: Pinless clamps attached to the AO-tubular external fiiator.
ZL7Development phaaez Shape and size cd the clamp Fig. 12: The Pinless clamp without the handles.
3.6 Development phaec: Connection b an A0 tubular tiator The missing link in the case of the Pinless clamp was its connection to an external fixator which could be used to join the individual clamps to each other.
The strength of the clamp arms was calculated to allow adequate flexibility. This flexibility “stores’ a certain amount of resilience which is required as soon as the tips sink into the bone. This is inevitable during the treatment period. The size and shape of the clamp arms have been adapted to the shape of the bone and its surrounding tissue.
In accordance with the principle of simplicity, only the simplest A0 tubular fixator was considered (one rod and one clamp per Pinless clamp). An independent connecting element between the clamp and the rod of the external fixator had to be developed in order to leave the surgeon free to place the clamp as he wished. A strut emerged as the optimal solution (Fig. 13). It was mounted on the articulated axle and could be rotated and fixed in any position using a nut This degree of freedom of the strut plus that of the simple, adjustable clamps of‘ the tubular system allowed the surgeon to connect the individual clamps with a straight rod from the A0 tubular fixator, regardless of their position.
Fig. 15: Three different clamp sizes. As far as mechanical strength is concerned, a stable fixation can be achieved using the small Pinless clamp. The large clamp is appropriate in cases in which a thick layer of soft tissue has to be accommodated, or when
s7
Frigg: Development of the Pinks system
swelling of the soft tissue is expected postoperatively. The asymmetrical clamp is intended for use on the mid-third of the tibia1 shaft since it permits bone fixation without damaging the lateral muscle compartment
Fig. 16: The asymmetrical clamp. Application of the small Pinless clamp in the mid-third of the tibia1 shaft anteromedially leads to a posterior displacement of the dorsal musculature. The pressure increases if the leg is supported dorsally.
3.8 Development phase:
optimimtion of the
Completion and
Pinlm clamp
This development phase aimed at optimising the clinically acceptable Pinless clamp. In addition to the development of new, simpler and safer handles, the clamps had to be improved with regard to their size, and their production simplified from a manufacturing point of view. It is of paramount importance that production be straightforward and inexpensive since there are countries in which the Pinless clamps may not legally be used twice. In line with these requirements, the hinge components were simplified and improved so that they could be manufactured in alummi ’ ‘urn. The utilisation of aluminium meant that the stargrind responsible for locking the hinges and the strut could be omitted. These were replaced by steel pins which are pressed into the ahrminium when the nuts are tightened. This type of hinge locking is not only easier to manufacture but also easier to use, since the surgeon chooses the setting without increments. The nuts and the steel pins can be reused since they no contact with the body and are not exposed to mechanical loads. For this reason, they are made of less steel. This guarantees optimal function when repeatedly. All metal parts can be reused.
have large stainused
The secret of the Pinless clamp is its arms. The better their quality, the better the clamp fixation on the bone. These are the only components which have direct contact with the bone and the soft tissues. They are made of titanium since this metal has better elasticity, is lighter and is tissue compatible. Even if alternatives were available, especially as far as cost is concerned, only the best may be used.
Fig. 17: Soft tissue impingement. 3.7.1 The lateral muscle compartment If the Pinless fixator is used for primary, short-term fiiation, it can be mounted anteriorly as is the case for all frame fixators. Penetration of the lateral muscle compartment is acceptable. Operation is considerably facilitated by this standard positioning of the clamps as it allows for the use of small or large clamps. 3.7.2 Incision If there is time and the instruments are available, then the incision for the clamp arms should be prepared in advance so that the tissues embraced by the clamp arms will suffer less damage. This is of particular importance if the clamps are going to be left in place for some time. The arms are designed so that penetration of the soft tissue is as straight as possible (as is the case when preparing the insertion of a Schanz screw).
Fig. 18: The small clamp.
S8
Fig. 19: The asymmetrical
clamp.
The Pinless fiiator at present under clinical testing is the result of three years developmental work During the course of this development there were long pauses in which the aims and the clinical relevance of the Pinless system were debated. In particular, the tendency towards the use of the unreamed nail held up the design and development efforts. Despite the established external fixation methods early nailing of open fractures, the Pinless fiiator secure itself a place in modem fracture treatment.
and will
Initial use of this new concept has shown that its areas of application are not yet clearly defied and new applications are constantly being discovered. The use of the Pinless fixator to lock an unreamed medullary nail is just one example. This combination would help less well-equipped clinics to achieve a biologically optimal, internal fixation even without the advantages of the latest OR equipment (e.g. image intensifier). Adaptation of the Pinless clamp for use in paediatric orthopaedics, on fractures of the forearm, hand or foot is in progress. Despite the many stages of development which have been completed, many still lie before us before the idea of the Pinless clamp can meet all clinical requirements.
3. The development of the Pinless external fixator: from the idea to the implant
R Frigg AO/ASIF Development Institute Clavadelerstr., CH-7270 Davos This paper describes the individual stages of development which led to the realisation of the Pinless clamp.
3.1.2 Have Pi
3.1h~uction 3.1.1 Req-ts The development group was given the task of developing a system which would permit fixation of a fracture of a long bone without penetrating the medullary cavity. The following requirements were used as guidelines ing the entire development phase: 1.
Simplicity The system foolproof.
To comply with all these requirements, it was apparent that an external fiiation system was called for. Any internal fixation procedure requires extensive surgery and would be inappropriate as a temporary fixation method. The advantages of the new Pinless system should lie in its simplicity and minimal additional trauma to the bone and soft tissue.
should
be
logically
constructed
systems been used before ?
An examination of existing patents showed that various “pin-less” systems had already been developed. All these systems held the bone between at least two metal points, the exception being Malgaigne’s “point” in which one side is held firmly in a strap.
dur-
and
2.
Rapid application Since the new system was to be designed as a temrxcrn, surgical procedure should be kept to
3.
Minimal damage to soft tissue and bone After temporary fixation of the bone using this system, the surgeon must still have every possible internal fixation proced~ open to him for the definitive stabilisation of the fracture.
4.
It should be possible for an inexperienced surgeon to apply the new system In disaster areas, war zones or less well-equipped hospitals, it is essential that a fracture be carefully immobilised in order to transport the patient to a place where extensive medical care is available.
5.
The number of instruments (and implants) should be kept to a minimLun The more instruments and implants used in a system, the more complex it becomes and the greater the number of possible errors. The system becomes confusing and instruction more difficult.
Fig. 1: J.F.Malgaigne’s
“point! (1840).
This development was followed by the Malgaigne which was used for fixation of patellar fractures.
Fig. 2: J.F.Malgaigne’s
“claw” (1643).
claw
s4
All the other pinless devices described had forfeited their simplicity on account of a complex, mechanical apparatus. When these systems were being developed, they were intended as final fixators for the stabilisation of fractures and in this sense their complexity is not surprising. The requirements were not the same at that time as they are now.
of the clamp would have to be quite considerable if it had to accommodate the soft tissue. Such a wide extension of the clamps led to even greater instability. Initially, the C-clamp with an adjustable tip on one side seemed very promising. The advantage of this clamp was that the function of the clamp could be actively adapted by the surgeon. The C-shaped arms could be made stiffer, i.e. with a minimum of elasticity. Once the clamp tip connected to the fixator arm was positioned on the bone, the adjustable tip could be pressed into the bone using an instrument. A special washer prevented slippage. This meant that the adjustable tip could subsequently only be moved in one direction.
Fig. 5: Two sizes of clamp with adjustable Fig. 3: J.H. Nicholson
tip.
et al. (1927). Femoral splint.
These existing internal “pin-less” fixation systems have no place in modem internal fixation since they contravene all the laws of biological internal fixation. Even if they only caused minimal damage to the bone, they would still destroy the soft tissue covering and thus the blood supply.
However, loading the clamp on one side only had the disadvantage that the tip of the fixator arm did not gain sufficient purchase on the cortex. The greater part of the force applied to the adjustable tip was taken up by the elasticity of the fixator arm, thus preventing the fixed tip from sinking into the far cortex. An attempt to hammer the tips into the bone failed because the opposite tip then sprang loose. In order to reduce the elasticity of the fiiator arm temporarily, special pincers were designed to hold the arm and transfer the force directly to the tips. This tension mechanism failed because once the pincers were removed the fiiator arm was no longer taut and the tips were inadequately anchored in the bone.
Fig. 4: Robert L. Judet, (1979).
3.2 Constructionphase: flexibleclamps
Fig. 6: Fixator arm pincer.
With the above-mentioned, unsuitable designs in mind, we began to develop a Pinless system which would meet modem demands. Initial attempts were made using a Cshaped spring similar to that suggested in the early 80’s by B.G.Weber for temporary, intraoperative bone fixation. However, it soon became apparent that the clamping force of the C-shaped spring was inadequate and that the size
3.3
Develapmentphase: rigid clamps
Somewhat disappointed, the idea of a preloaded, springlike clamp was abandoned and work on construction of a rigid clamp commenced. The fiiator arm was constructed in the form of an I-beam-support. The clamps were
s5
Frigg: Development of the Pinks system
anchored in the bone by rotation of two adjustable tips. The bone fixation achieved was excellent. In addition to the clamping power achieved by rotation of the trocar points, the anchorage of the tips in the cortex was improved because the trocar points could drill into the bone.
sites of a lag screw. The elastic nature of the forceps allows the surgeon to feel whether the tips are slowly slipping, sinking into the bone, or pushing the fracture fragments apart
Fig. 9: Pointed reduction forceps Fig. 7: Rigid clamp. The price of good anchorage in the bone was the increased difficulty in mounting the clamp. Although it was possible, after some practice, to mount the clamp under direct vision, without it, it was extremely difficult. Feedback from the clamp to the surgeon’s hand was poor making it impossible for him to tell whether the tips were well positioned on the bone or not. This meant that it tended to slip off. Reservations about the consequences of bone resorption around the tips during the course of treatment leading to a loosening of a rigid clamp with no elastic preload, led us to abandon this type of fixation.
3.4 Development phase: The forcepa principle
To use the forceps, the bone is fit properties described above are even more the clamp is to be mounted on the bone the soft tissue). Tests using large and forceps confirmed that the problem of solved.
exposed. The indispensable if blind (through small reduction the clamp was
3.5 Devehpment phase: Elaboraiion of the forcepa
pinciple The main task of further development was now to simplify the reduction forceps and integrate them into an external fixation system. Existing reduction forceps were altered so that the handles could be removed. This only led to a partial reduction in volume because part of the handles had to be left in place in order to accommodate the catch.
The investigations carried out with the rigid clamp demonstrated that the success of the Pinless external fiiator did not depend on the rigidity of the fixation but rather on the simplicity of its application. In other words the surgeon must be able to feel whether the clamp is properly positioned or not. A clamp should match the extended index finger and thumb of the surgeon, since these digits form the best “clamp”.
Fig. 10: Pointed reduction forceps with removable handles. To reduce the volume of the forceps down to an acceptable size, the locking catch and the hinge had to be redesigned. A special hinge was made which could be locked in any position using a nut or screw. The arms of the forceps were pressed into holes in the hinge and fixed in place. Opposite these holes, two further holes were made into which the handles could be inserted. Fig. 8: The best “clamp”. A temporary fixation is usually simply and successfully performed using reduction forceps. By feeling the tips of the forceps are placed at the optimal insertion and exit
S6
Fig. 13: Pinless clamp with detached strut
Fig. 11: The first Pinless clamp. Having mounted the Pinless clamp, easily be completely removed.
the handles
could
Fig. 14: Pinless clamps attached to the AO-tubular external fiiator.
ZL7Development phaaez Shape and size cd the clamp Fig. 12: The Pinless clamp without the handles.
3.6 Development phaec: Connection b an A0 tubular tiator The missing link in the case of the Pinless clamp was its connection to an external fixator which could be used to join the individual clamps to each other.
The strength of the clamp arms was calculated to allow adequate flexibility. This flexibility “stores’ a certain amount of resilience which is required as soon as the tips sink into the bone. This is inevitable during the treatment period. The size and shape of the clamp arms have been adapted to the shape of the bone and its surrounding tissue.
In accordance with the principle of simplicity, only the simplest A0 tubular fixator was considered (one rod and one clamp per Pinless clamp). An independent connecting element between the clamp and the rod of the external fixator had to be developed in order to leave the surgeon free to place the clamp as he wished. A strut emerged as the optimal solution (Fig. 13). It was mounted on the articulated axle and could be rotated and fixed in any position using a nut This degree of freedom of the strut plus that of the simple, adjustable clamps of‘ the tubular system allowed the surgeon to connect the individual clamps with a straight rod from the A0 tubular fixator, regardless of their position.
Fig. 15: Three different clamp sizes. As far as mechanical strength is concerned, a stable fixation can be achieved using the small Pinless clamp. The large clamp is appropriate in cases in which a thick layer of soft tissue has to be accommodated, or when
s7
Frigg: Development of the Pinks system
swelling of the soft tissue is expected postoperatively. The asymmetrical clamp is intended for use on the mid-third of the tibia1 shaft since it permits bone fixation without damaging the lateral muscle compartment
Fig. 16: The asymmetrical clamp. Application of the small Pinless clamp in the mid-third of the tibia1 shaft anteromedially leads to a posterior displacement of the dorsal musculature. The pressure increases if the leg is supported dorsally.
3.8 Development phase:
optimimtion of the
Completion and
Pinlm clamp
This development phase aimed at optimising the clinically acceptable Pinless clamp. In addition to the development of new, simpler and safer handles, the clamps had to be improved with regard to their size, and their production simplified from a manufacturing point of view. It is of paramount importance that production be straightforward and inexpensive since there are countries in which the Pinless clamps may not legally be used twice. In line with these requirements, the hinge components were simplified and improved so that they could be manufactured in alummi ’ ‘urn. The utilisation of aluminium meant that the stargrind responsible for locking the hinges and the strut could be omitted. These were replaced by steel pins which are pressed into the ahrminium when the nuts are tightened. This type of hinge locking is not only easier to manufacture but also easier to use, since the surgeon chooses the setting without increments. The nuts and the steel pins can be reused since they no contact with the body and are not exposed to mechanical loads. For this reason, they are made of less steel. This guarantees optimal function when repeatedly. All metal parts can be reused.
have large stainused
The secret of the Pinless clamp is its arms. The better their quality, the better the clamp fixation on the bone. These are the only components which have direct contact with the bone and the soft tissues. They are made of titanium since this metal has better elasticity, is lighter and is tissue compatible. Even if alternatives were available, especially as far as cost is concerned, only the best may be used.
Fig. 17: Soft tissue impingement. 3.7.1 The lateral muscle compartment If the Pinless fixator is used for primary, short-term fiiation, it can be mounted anteriorly as is the case for all frame fixators. Penetration of the lateral muscle compartment is acceptable. Operation is considerably facilitated by this standard positioning of the clamps as it allows for the use of small or large clamps. 3.7.2 Incision If there is time and the instruments are available, then the incision for the clamp arms should be prepared in advance so that the tissues embraced by the clamp arms will suffer less damage. This is of particular importance if the clamps are going to be left in place for some time. The arms are designed so that penetration of the soft tissue is as straight as possible (as is the case when preparing the insertion of a Schanz screw).
Fig. 18: The small clamp.
S8
Fig. 19: The asymmetrical
clamp.
The Pinless fiiator at present under clinical testing is the result of three years developmental work During the course of this development there were long pauses in which the aims and the clinical relevance of the Pinless system were debated. In particular, the tendency towards the use of the unreamed nail held up the design and development efforts. Despite the established external fixation methods early nailing of open fractures, the Pinless fiiator secure itself a place in modem fracture treatment.
and will
Initial use of this new concept has shown that its areas of application are not yet clearly defied and new applications are constantly being discovered. The use of the Pinless fixator to lock an unreamed medullary nail is just one example. This combination would help less well-equipped clinics to achieve a biologically optimal, internal fixation even without the advantages of the latest OR equipment (e.g. image intensifier). Adaptation of the Pinless clamp for use in paediatric orthopaedics, on fractures of the forearm, hand or foot is in progress. Despite the many stages of development which have been completed, many still lie before us before the idea of the Pinless clamp can meet all clinical requirements.
