4. Biomechanical evaluation of the Pinless external fixator

s9

4.

Biomechanicd evaluation of the Pinless external fixator

G.Moe Stenel, R.Friggl,

LJ.Schlegel’,

M.Swiontkowski2

4.2In~ction

* AO/ASIF Research Institute Clavadelerstr. CH-7270 Davos Platz

4.21 The &s&al

2 Harborview Hospital Seattle, Washington, USA

4.1 Abstract In open fractures especially in those with severe soft tissue damage, fracture stabilisation is best achieved by using external fixators. There are some intrinsic complications which occur during classical external pin fixation. To overcome the problem of pin track infection and vascular damage from drilling, the Pinless external fixator was developed. It is based on the idea of a forceps with trocar points, which only penetrate the bone cortex superficially. The function of the device was tested in two mechanical trials and two in vitro tests in which one pinless clamp was put under a controlled load of 50 N, 150 cycles/day and studied over a 5 week period in sheep. The loads and time range of the experiment were chosen to simulate a temporary fracture stabilisation in a patient not bearing weight. The main question to be answered was whether the Pinless external fixator would be able to maintain stable fixation. Furthermore, it was to determine the changes at the trocar-to-bone interface. The clamp was found to maintain 72 % of the initially applied clamping force after 5 weeks of in vivo application and it was found to be tight at removal. Some decrease of clamping force was found during the fit 20 days and then the force tended to level off. There was no slippage nor did the clamp penetrate the cortex. There were no obvious signs of infection around the trocar-holes and in the bacterial tests no pathological cultures were grown. Histology revealed very localiied bone reactions, the indentation caused by the trocar tips being only 1.2 mm deep. The study concludes, as far as could be ascertained from these tests, that it is safe to use pinless external fixation for temporary fracture fixation. Conclusion: Pinless external fixation offers stabilisation fractures with minimal disturbance of the bone.

of

extemal fixation method

Since external fixation (E.F.) is much less disturbing than internal fixation regarding the wound, fracture site and bone vascularity, and since it is relatively simple to apply, it is a practical solution for the initial treatment of open fractures [70]. However, having attached the classic external fixation device, it is difficult to change the procedure because of some risk of infection. Bone healing following external fixation may be slow so that a change of treatment method, e.g. from E.F. to medullary nailing, may be indicated. 4.2.2 History of E.F. development External fixation for fracture stab&&ion has been known since the last century, beginning with the idea of the patella claw of Malgaigne in 1843, [91]. In 1897 Parkhill described the fit external device for the fixation of long bones, [87l. External fixation experienced variable popularity until the 1960’s when positive clinical results with the Hoffmann and the modified Vidal-Adrey frame were reported, [87, 30, 61, 52, 51, 57, 601. Primarily in open fractures or in fractures with severe soft tissue injury, external fixation is now an accepted choice of treatment [92, 50, 66, 26, 91. Primary stabilisation of such fractures with internal fixation devices (conventional plate or reamed nail) has shown high complication rates, [41,

Ml.

Fig. 1: The principle of a classical unilateral external fiiator. Current external fixators normally use Steinmann pins or Schanz screws, which are inserted through the bone and which connect the bone to the outside instrumentation, namely the clamps and rods, see Fig. 1. The implants are attached at a distance from the fracture site to allow easy access for the management of soft tissue injury, stabiity for the healing of these tissues, and rigid early fracture fixation, which allows for patient mob&&on.

Author

]57l

Karlstroem [52] K=mpen Allerton [4]

Yeal-

FX.no.

FX.type

Pin tract uroblems major minor

Velazco [90]

Karlstoem [Sl] Schmidt [84]

Heiser [44]

Gershuni [36]

Edwards [26] Etter [2fl

Aho 131

Kimmel [55]

Benum [lo]

1985

1984 1984

1983

1983 1983

1983

1983 1983 1983

1983

1982

1982

48 42

19

288 16

40

111 34

20

33

246 44

79

27

Vidal-Adrey Vidal-Adrey/Hoffmann

A0 Unilat

Hughes Unilat. Vidal-Adrey

Taylor

EF/EF c MinOst.

Orthofi AO-LJnilat/Wagner

Vidal-Adrey

Hoffmann Flex.half frame

Hofmann/ Roger-Anderson

A0

Hoffmann A0

31% 21%

12%

26% 5%

33% 14%

4% 100%

55%

5% 15%

8%

1%

Delayed! nonunion

5%

20%

13%

7% 18%

55%

98%

malunion

26% 55% 39%

27%

Sec.IF

11% 9%

Healing time (months)

10

5

6

5

7

9.5

6%

10

4%

3%

7 6

52%

4%

5 11

5%

19%

9P7 8

65% 6% 41%

11%

>6

5

8

9

6

10

11%

30%

14%

39%

22%

38% 9%

38%

78%

86%

12%

19% 6% 3%

6% 23%

12%

14%

25%

46%

11% 9% 8%

26%

Vidal-Adrey

4%

4%

28 Vidal-Adrey Pins in plaster 32% 42%

32%

78%

43%

1975 23 38 Portsmouth

7%

108%

1979 1981 38

58% 4%

De Bastiani [S] Chan [19]

1985 1985 75

Hoffmann

31%

21%

1981 Vidal-Adrey

13%

50%

Scharf [86] cannon 117)

1986

66 42 171

A0

P4 Vidal-Adrey

8%

Court-Brown [ 231 Hammer [42] 1986 1987 1988

42

Vidal-Adrey

Edge 50 Vidal-Adrey

32% 9%

Behrens [B]

1988

32

AORingfii

18%

Rommens [Bl] Clifford [ 22) Edwards (251 1988

30

13%

of tibia1 fractures, mainly open and complicated fractures, listed

19%

42%

Genelin (351 Nesbakken [72]

1988

119

+ Min.Ost

Reith [79]

1989

1984

Rommens [82]

Table 1: Complication rates from different authors who have used external fixation for the stabiition chronologically. (SEC.IF = secondary internal fixation.).

Mw Stene: Biomechankal evaluation

s 11

4.2.3 Problems If the transfixation pins remain in place for a long time, intrinsic problems with the external fiiators are likely to arise [22, 821. The main problems are pin loosening, pin track infection and osseous healing disturbances (nonunion, delayed union and malunion), [40] (Table I). Interestingly, the same classical problems have always been associated with external fixation devices [71, 39, 69, 87, 821, although efforts have been made to solve them, e.g. by determining the mechanical properties of different frames, [21, 16, 8, 6, 9, 14, 19, 22, 29, 30, 46, 46, 65, 691, preloading pins axially, [43, 691 or radially, [35, 12, 481 to prevent pin-loosening and infection, and dynarnisation of the fracture site for optimisation of healing conditions after primary rigid stabiIisation, [17, 7, 54, 53, 85, 6, 371. Drilling the bone damages vascularity [63]. It would be an advantage to have an alternative fixation device for intmoperative reduction, distraction and temporary stab&&ion. Healing disturbances: The high incidence of these main complications and especially the long healing time for these complicated fractures may indicate that a secondary procedure of internal fracture stabilisation should be carried out once the soft tissue has healed. This could be advantageous for fracture healing and beneficial for the patient as well. Pin Loosening: Pins of the E.F. are known to lose their tight fit within bone with time. This complication is of less importance in respect to the concomitant loss of fixation stability than to the link between loss of stable anchorage and related pin track infection. Pin loosening has been observed to occur in up to 60% of the pins in clinical practice [19, 251. Implant loosening is probably due to micromotion and instability of the implant, leading to bone resorption [76, 89, 32, 77, 481, rather than simple mechanical overload [47]. Tmnsfixion: The penetration of the pins&hanz through the bone is one of the major problems: 1.

2.

screws

Demanding Application: Pins which transfix bone often require careful application, that is to say, the pins must be aligned so that fixation to a connecting rod is possible. Problems ~&ted to change of treatment: If the pin to bone connection loosens due to bone resorption, the periosteal side and the endosteal side of the bone are connected. This is a problem when a transcutaneous infection is present - the pins then provide a path whereby a minor superficial infection can lead to deep track infections, osteitis, infection (pin bone osteomyelitis) [38, 511. The situation is even more complicated because in the case of pin loosening the pmbability of transcutaneous infection is increased [56, 25, 601. Not only are these infections undesirable in themselves, but they are even more pernicious when the infection is in the medullary cavity and a revision fmm external fixation to medulIary nailing is necessary. In thii case a medullary infection would be a severe problem [65, 64, 90, 501. It is generally accepted with the classic external fiators that one has to wait for the trocar-holes to close [3] and for any signs of infection to disappear before medulIary nailing can be carried out in order to avoid the major

risk of deep infection. Varying time intervals between removal of the external fiiator and insertion of the nail have been discussed [79,65, 64,9O, 50, 121. If a pin track infection is present nailing wilI be compmmised, [3, 65, 6% 9Q

501.

4.2.4 Development of the Tinless” E.F. The idea of a clamping device for bone fixation is not new. The patella claw from Malgaigne, [91] was the first known external bone fixation device. Later different surgical instruments with trocar points penetrating the bone cortex superficially were developed and used. As early as 1944 a guiding device for the insertion of surgical pins was presented by Pincock [78], and in 1960 a reduction device for femoral neck fractures was developed 1591. The Weber reduction forceps and are a standard instrument in are well-known orthopaedic surgery today. Mennen [68] has recently presented a Clamp-On Plate, and with the new era of biological fixation (indirect reduction techniques) Ganz [31] has invented a clamp for the fixation of unstable Pelvic fractures. The Halo-device for the fixation of cervical spine injuries also depends on trocar points with only superficial penetration of the cortex [74, 151. The Pinless external fixatorl was developed to mi+nise the occurrence of pin track infections and vascular damage from drilling. The basic idea of pinless external fixation2 came fmm Swiontkowski3 and the development of the pinless clamp4 was done by Frigg5 and his team. It is based on the idea of a clamp with two trocar points which penetrate the bone cortex only minimally, similar to the bone forceps used in surgery for oFtive reduction of fractures (Weber clamps) and to the principle of the Halofixator used for neck fractures. The clamp can be preloaded due to the elastic section included in the arms (Fig. 2), which gives it more holding power even if some subsidence were present The clamp is constructed in the shape of forceps with removable handles. In one plane it has a 3600 f--moving pin which can be connected to the existing A0 clamps and tubes. This gives freedom for alignment in more than one plane (Figs 3 and 4) whilst assembling the whole frame. During application, the pinless clamp grips like a forceps onto the bone, then it has to be moved back and forth a couple of times to drive the tmcar points a little into the outer part of the cortex; it can then be preloaded by pressing the handles together when tightening the lock Then the handles can be removed (Figs 3 and 5.

’ Pinless external fixator, pinless clamp or clamp will be used as name for the constructed

2 Pinless external

device.

fixation will be used as the expre&on

method of E.F. using the Pinless external fixator. 3 Harbour View Hospital, Seattle 4 Now in clinical testing by the AO/ASIF group 5 AO/ASIF Development Institute, Davos

for the

s 12

Fig. 2: Single pinless fixation clamp as used for the experiments. The thinner section on the arms makes them elastic.

Fig. 5: briefly

are drilled

43Aimofthepmsentstudy To our knowledge there is no available implant on the market comparable to this newly constructed device. The aim of this studi was to test the be&&our of the Pinless external fiator in v&o and in &o before it was applied to human clinical cases. The Halo, device is the most comparable system, but since ‘the Pinless external fixator is intended for application on the long bones, we have very different fGtctiona1 loading con&ions. The specific aims of the investigation can be divided into biological and mechanical

Fig. 3: Schematic diagramme of handle removal; note how th< connection pin can be differently positioned.

Mechanical: The decrease of clamping foyce (= preload decrease) of the clamp applied in ;ivo should be determined over a rele\fant time period. This should give an indication of the possible- need for retightening. kn the handling of the Halo fiiator, it is a common procedure to retigh?en [74, 15, 84, 34, 931.

Biological: We need to determine

the biological reaction of soft tissue and bone by putting the pinless clamp under clinically relevant load. This is important because the holding power of each clamp relies on the contact between the tmcar Points and the bone cortex.

Fig. 4: Pinless external fixator. The experimental clamps mounted on a plastic bone to show the principle of pinless external fixation.

Since we do not expect to have a more stable combination at the trocar-bone interface of the pinless clamp than we have in the pin-bone interface of the Schanz screws, it is to be expected that we will also see loosening at the trocar-hole of the pinless clamp after a certain period of time due to bone resorption resulting from micromotion. The question is how quickly will bone resorption around the trocar tip reach a level where it will lead to significant instability allowing the clamp to slip ? We discuss the indications of the Pinless external fixator primarily as a temporary fixation device, especially for open fractures or tibia] f&ures with severe soft tissue iniu&. The uinless fiiator would not onlv allow safer fixation’ &relation to the medullarv cavity, bu; would also be especially expedient in catast&phic &uations for easy, quick and safe fracture fixation. As a temporary device for inttaoperative fixation with the distractor, as reduction forceps or as a temporary locking device in medullary nailing pinless external fixation would offer the advantage of minimal bone tissue disturbance and avoid the need to drill.

Moe Stene: Biomechanikalevdua tion

4.4 Maw

s 13

and methods

4.4.1 Common aspects of the experimental The main question was to find a model be able to control the exact amount of pinless clamp. A pneumatically operated which permits application of a cyclical clamp, was used for controlled loading.

4.4.2 Calibration and mechanical model used in which we would loading put on one external fiiator rod, bending load to one

testing

Calibration of the strain gauges and the relative displacement of the trocar points, and two mechanical tests were done using the INSTRON 4302 testing machine. The mechanical tests were performed to find out if the pinless clamp would show any relaxation or plastic deformation in the arms during loading. Calibmtior~ The pinks clamp was tightened in a set position so that the arms had an opening between the trocar points of 20 mm (the sheep tibia has a diameter of approximately 20 mm). It was mounted in specially designed holding devices in the INSTRON machine, so that it could be loaded in tension by pulling on the trocar points, thus simulating the loading of the clamp when it is applied on a bone (Figs 8 and 9). For calibration, each pinks clamp was loaded from zero to 200 N several times and the following parameters were recorded: strain in the clamp arms at 10 and 200 N and the distance between the trocar points at 10 and 200 N loading. We will call the “opening up” of the clamp under load, the relative displacement of the two trocar points. Hence, for each force applied to the clamp arms the measured sbain values and the corresponding displacement between the trocar points were recorded.

Fig. 6: The experimental model: The active external fixator rod consisting of a pneumatic cylinder fixed to the bone by two Schanz screws. By regulation of the air pressure and a timer the amount of loading put on the pinks clamp and number of cycles could be controlled. Two strain gauges were mounted on each arm of the pinless clamp in a longitudinal direction and connected to a fuU bridge. This positioning of the strain gauges permits measurement of bending in the clamp arms, while torsion and temperature are controlled electrically. Bending force is the main force in the arms of the pinless clamp when we apply and tighten it onto the bone; it is a good measure for the clamping force of the clamp. If we have some small torsion due to the fact that the trocar points might not be exactly in the same plane pointing towards each other (from construction)- this will also be compensated for in the full bridge connection.

Fig. 7: The position of the strain gauges. Note also the direction of loading of the clamp by the active fiator rod.

Fig. 8: Experimental set up for calibration and for the two mechanical tests in the INSTRON testing machine. To ensure that small changes in the set distance between the trocar points would not influence the calibration, the same procedure was repeated on one of the clamps which was initially fixed open at 22 mm.

s 14

iii)

In vitro application of pinless clamp for scanning electron microscopy In order to see a pinless clamp trocar-hole in three dimensions, small cortical bone cubes with trocar-holes were prepared for scanning electron microscopy (SEM). The clamp was applied on a sheep cadaver tibia three times. It was just pressed into the bone, moved back and forth 8 or 16 times, then it was preloaded and tightened. Each trocar site was cut out as a little bone cube, fixed in ethanol, and fat was removed by xylene. Then the specimens were air dried and sputtered with gold for the scanning electron micruscope.

Fig 9: A closer view of the mounting of the clamp in the holding devices and schematic representation of the way in which the relative displacement of the trocar points could be measured very accurately with an extensometer. Static test: The pinless clamp was mounted the same way as for calibration. The clamp was loaded to 24Xl N and the position was “frozen”; this means that the distance after relative displacement of the trocar points was held constant. The change of force in the clamp arms was recorded over 7 days as the corresponding change in strain. Any decrease in the clamp force would mean a relaxation or deformation in the clamp arms. Dynamic test: The pinless clamp was again mounted in the testing machine as for calibration. Then it was loaded cyclically 46 cycles/hour between 100 and 200 N and the corresponding relative displacement values were recorded for 22 hours. (These values were chosen to be sure that the clamps were loaded with a functional load or greater). Any increase in displacement would mean deformation in the clamp arms. 4.4.3 In vitm experiments Three in vitro experiments were performed. Experiments i) and ii) were performed as a pilot study for the in viva studies with the purpose of determining the mechanical behaviour of the pinless clamp applied in vitro; more specifically, to measure the decrease of clamping force over time with the clamp applied on a cadaver bone. 9

In vitm static: One pinless clamp with mounted strain gauges was applied on a cadaver sheep tibia and the strain was measured over 8 days. The bone was kept moist with Ringer solution (Fig. 10). In vitro dynamic: One pinless clamp with mounted strain gauges and two 4.5 mm Schanz screws (as fixation pins) were applied on a cadaver sheep tibia and connected using the active external fixation md. The clamp was then loaded cyclically for 6 days by the pneumatic cylinder with a force of 50 N, 150 cycles/day and strain measurements were recorded. At the end of the experiment the bone was fixed and prepared for histology.

Fig. 10: In vitro static test Note the silicon tube used for protection of the strain gauges. 4.94 In viva study RxperimentaI set up: Three Swiss alpine sheep (019, 059, -28), 6 years old, were used as experimental animals (permission GR 4/l990). They all had one pinless external fiiator and two Schanz screws (for fixation of the rod) applied on the mediil side of the right tibia and connected by the active fiiator rod. The operations were performed under sterile conditions with the sheep in Halothane anaesthesia. During application every clamp was moved back and forth 8 times by the handles so that the trocar points cut themselves a little into the bone before tightening. On each sheep the clamp was then loaded 50 N, 150 cycles per day by the frame. All the sheep were kept for 5 weeks. Observations and afterrare: The sheep were kept in safety sling-4 after the operation so that they were able to stand and bear full weight. Daily strain measurements were performed using the HBW Digital Strainmeter DMD 2OA. Every five days the insertion sites (pinless clamp and Schanz screws) were cleaned with chlorhexidine, the crust removed and new sterile dressings ’ Loosely applied slings to prevent overload when lying down and standing up while allowing for a convenient sleeping position 7 Hottinger-Baldwin

Messtechnik

s 15

Moe Stene: Biomechanical evaluation

applied. Weekly X-rays were performed and fluorochrome labelling given (see observation scheme, Fig. 11).

X-ray

OP

x0

1

I

x-ray

x-ray

X-ray

CG xr

7

bacterial swabs taken

I

1

I”““I’~~~~~I”~~~~l~~~~“l’~~~“I 0 T

T’

T

*T

Fig. 11: Observation CG = calcein green.)

5

T3

T 4T

scheme.

(X0

weeks

Din sitemre =

xylene

orange; J

Immediately after killing the animals, bacterial taken fmm all insertion sites (clamp and pins)g.

swabs were

Preparation for histology: The specimens were cleaned, fixed, and dehydrated in ethanol. They were then taken through xylene, embedd e d in methylmethacrylate, and finally cut into sections with the saw9. After grinding some of the sections were surface stained with basic fuchsin and the rest were left unstained for micmradiography and fluorescence microscopy.

43 Results 4.5.1 Calibration and mechanical

testing

Fig. 12: The diagramme shows the corresponding strain values in the pinless clamps to the loading force of the clamps. When the pinless clamp is loaded, either when applied to a bone and tightened with a preload, or loaded as in the calibration setups, there will be an “opening up” of the clamp: the distance between the trocar points will increase. This means we will have a relative displacement of the two trocar points, the value being dependent on the loading force. This is elastic deformation of the arms of the pinless clamp. As shown in the displacement calibration, loading of the clamp with 100 N will increase the distance between the trocar Points by about 1.5 mm (Fig. 13).

Calibration: Four clamps were calibrated, three for use on the sheep and one for the mechanical tests.

Displacement

The sensitivity of the strain gauges on each of the four clamps was found to be very similar (Fig. 12). This indicates that the gauges were mounted similarly and that the strain readings were reliable. An enlargement of the gap opening, 2 mm wider (opening set to 22 mm), demonstrated that such a variation had no observable effect on the calibration coefficients. The calibration process should, therefore, be valid for all possible opening widths in the range of sheep tibia1 diameters. The force calibration curve indicates a high sensitivity, microstrain per Newton, of the instrumented clamp.

Calibration

50 Pulll~g’kc:.

160 (N)

CllPNo. -1

-2

-a(T2mm)

-5

-.

25

I

Fig.

13: The

displacement

diagramme

shows

of the trocar

points

the corresponding to loading force.

relative

Static test: The results of the static force relaxation test in which the clamp is held under tension on the trocar points at a constant distance after relative displacement for 7 days can be seen in Fig. 14. There were no substantial changes in relative tensile force during this time. Since no change of force in the clamp arms could be measured, it is assumed that no plastic deformation or creep of the material in the arms under static loading occurred in these 7 days.

8

The

bacterial

microbiological

swabs

were

department of

9 Leitz Siigemicrotome

1600

cultured

and

analysed

Ziirich Animal Hospital

by

the

S 16

detectable (Fig. 16).

Clip force with displacement held constant

decrease

of the preload

Clamping 150Clamping

under

these conditions

Force of Pinless Clip in vitro static

force (N)

1x100 ---75

~

50 25 0 0

Fig. 14: Force measured in the clamp arms with the distance between the trocar points held constant (3 mm), which means the clamp was loaded statically in tension for 7 days. No change of force in the clamp arms could be measured. Dynamic test: Within the framework of the dynamic mechanical set up, we measured the relative displacement between the trocar points at 100 N and at 200 N during cyclic loading of the clamp. We measured an increase in the relative displacement of the trocar points at both the minimum and the maximum pulling force, which indicates that the clamp is slowly opening under dynamic loading. Fig. 15 shows this increase in relative displacement measured at minimum load. This increase of the relative displacement is due to stress relaxation or plastic deformation in the arms of the pinless clamp. From the graph we can extrapolate the increase to be less than 0.1 mm in 1000 cycles. Using the displacement calibration curve (Fig. 13), we see that this equals a force of less than 10 N.

3

I

5

6

7

5

Fig. 16: Clamping force measured in the arms, with the pinless clamp applied on a cadaver sheep tibia and without any functional loading of the clamp. In vitro dynamic: Clamping force measurements: In the second in vitro experiment the pinless clamp was dynamically loaded by the pneumatic cylinder. In a situation in which the preloaded clamp is being loaded, some micromovement of the tips is to be expected. Siice the trocar points of the clamp are self-cutting, it may be possible for the trocar tips to drill themselves deeper into the bone cortex due to some movement. This could diminish the clamping force of the pinless clamp. If this is the case it should be revealed by this in vitro experiment It was observed that even in the case of a dynamically loaded clamp there was no change from the initially applied clamping force with time (Fig. 17).

Clamping Force of Pinless Clip in vitro dynamic

of Trocar Points loading

2

Time (Uayr)

7

Relative Displacement at 1OON during cyclic

I

of clip

2ooClamplng

force (N)

150’

50 -

0

0

-_

1

1

3

4

5

6

Time (days)

J Fig. 15: The diagramme shows the relative displacement of trocar points during cyclic loading of the clamp between 100 and 200 N (46 cycles/hour); the displacement measured at 100 N. We see an increase in the relative displacement of the trocar points during the 22 hours of cyclic loading. 4.5.2 In vitro experiments In vitro static: The static in vitro experiment, with the pinless clamp applied on the bone with preload but without any loading of the clamp itself, should reveal any clamping force changes due to time-dependent mechanical properties of bone tissue. Measurement of the clamping force shows no

-I Fig. 17: Clamping force measured in the arms, with the pinless clamp applied on a cadaver sheep tibia. The clamp was dynamically loaded with 50 N, 150 cycles/day. HistoQical observations: Histology sections from the dynamic experiment were prepared. Cross sections of the bone shaft were made through the trocar-holes. These sections from in vitro application of the pinless clamp show the mechanical damage which occurs after application of the clamp and subsequent loading for 7 days. The overview picture (Fig. 19) shows that the trocar-hole can be seen as a small triangle, which has been cut by the trocar point into the bone, and we see the size relationship of the

Moe Stene: Biomechankal evaluation

s 17

trocar-hole to the bone cortex. This relationship should be compared to the size of the trocar point shown in Fig. 18. The greater magnification of the trocar-hole in Fig. 20 shows the damage to the bone resulting from the tip of the pinless clamp more clearly. Close to the hole the bone tissue is crushed. A little deeper the bone tissue is disturbed by microcracks and fractures between the osteons and some microfragments have been created. Some osteons in this area have been irreversibly deformed. This damage, possibly resulting from preloading and tightening of the clamp, is more pronounced on one side of the trocar-hole near the edge. In fact it is concentrated on the medial side of the hole (the sides towards each other, inside the clamp) and more or less intact; undisturbed bone is seen on the opposite side of the trocar-hole.

Fig. 18: Trocar tip of pinless clamp. Magnification x 16.

Fig. u): Histology of in vitro specimen. Section cut through the central part of the trocar-hole of the pinless clamp. Note the crushed bone inside the hole and the distu&ed (microcracks, microfragments and irreversible deformation) cortical bone tissue on the medial side of the trocar-hole. Magnification x 48.

Fig. 21: SEM. No rocking movements were applied during application of the clamp. The trocar point can be seen to have made a clean impression in the bone tissue. Magnification x 70. scanning electnm minoscopy: The trocar point can be seen to press itself cleanly into the bone, if the pinless clamp is not moved back and forth a few times before tightening the lock In Fig. 21 the tip of the pinless clamp can be seen as an impression with the exact shape of the trocar point.

Fig. 19: Histology of in vitro specimen. Overview picture of the pinless clamp trocar-hole. The size of the hole should be compared to the cortex thickness and to the sire of the trocar in Fig. 20 which has the same ‘magnification. MaPnification x 16.

In the specimens involving back and forth motion of the clamp during application, the trocar-hole can be seen as a round hole partially fiied with micro bone fragments (Figs 22 and 23). It can actually be seen that the trocar points in these specimens have drilled themselves a little into the cortical bone. On the edge of the hole some microcracks can be seen, but the wall is obviously still intact.

s 18

or on the part of the loading cycle during measurement could influence the strain values. The measurements were taken as mean values of the daily interval of values. The clamping force drops on an average 25 N, this is 28%, (22X, 24% and 39%) of the mean value respectively and means a decrease of 0.36 mm in the distance between the trocar points. The main prelad decrease is seen in the first 20 days; with time the curves level off. Clamping

Force of Pinless Clip in vivo

Fig. 22: SEM. The pinless clamp was moved back and forth 8 times at application. The tmcars have cut into the bone and formed a round hole. Magnification x 85. IO

0

-

ShHP

ON

40

30

-

smep osv

-

Sbvvp

-?a

Fig. 24: Clamping force measured from pinless clamps in the experimental sheep. A more prominent decrease is seen in the fit 20 days, then the force tends to remain fairly constant.

Fig. 23: SEM. The pinless clamp was moved back and forth 16 times at application. This greater magnification shows the intact wall of the trocar-hole. Note the minimal difference between the size and appearance of this hole compared to the hole created from 8 motions (Fig. 22). Magnification x 85. No remarkable difference was found between the trocarholes created by 16 instead of 8 motions of the clamp during application, compare Fig. 22 with Fig. 23. 4.5.3 In viva study

Fig. 25az The pinless clamp connected frame.

to the active fixator

4.5.3.1 Clamping force measurements Fig. 24 shows the clamping force change with time for the three pinless clamps applied in vivo. The clamps were tightened during the operation with the same amount of preload (measured on a strain gauge). The differences in the amount of initial clamping force seen on the result curves in Fig. 24 can be explained by the fact that the first amount of applied preload drops the moment the clamp has been tightened and the surgeon’s hands are loosening their grip on the handles. At this moment the lock system is adjusting itself. This initial drop in preload happens quickly and then it stabilises. The first measurements shown here were taken approximately 1 hour Post-op. The variation in the curves may occur for different reasons. The strain gauges were very sensitive which means that small changes in the amount of weight-bearing by the sheep

Fig. 25b: A close-up of one clamp arm. Figs 25 a and b show the Pinless clamp applied on the sheep tibia 2*$4weeks post-op. The Position of one strain gauge can

Mw Stene: Biomechankal

evaluation

s 19

be seen. Note the clean, dry insertion sites with no signs of infection. 4.5.3.2 Clinical observations All insertion sites had healed after 2 weeks. There were no clinical signs of infection (Fig. 25). Two bacterial swabs from a growth of staphylococcus 42 probes taken showed epidermis. No signs of microorganisms or infection could be found using the microscope either on the swab or on the histology sections. All three pinless clamps were clinically still tight at the time of removal. No signs of slippage could be seen. In the freshly explanted bone a collar of external callus which had developed around the trocar-hole, especially at the posterior site, could be seen macroscopically. The trocarhole itself was seen as a very slight indentation in the cortex. 4.5.3.3 Radiological observations As can be seen on Fig. 26, a small faint radiolucent zone and callus formation can be detected around the insertion sites between the third and fourth week post-op. 4.5.3.4 Histological observations Overview analysis: For histological purposes sections of tibiae were cut as cross sections of the tibia1 shaft and through the trocar-holes. Three sections were used for the evaluation: the central cut we called position SO and the cuts on each side S1 and S+l (Fig. 27). A

Fig. 27: The diagramme shows how the three section types looked. SO is the central cut through the trocar-hole. S-l and S+l are the side cuts. Note how the triangular shape of the trocar-hole can be seen in the central section (SO). The side cuts (S-1, S+l) are then either smaller triangular-shaped cuts, or further away from the tip, we only see part of the resorption groove. Microscopically each trocar-hole of the pinless clamp can be seen as a triangle which has been cut into the bone cortex by the trocar points (Fig. 28); this we had already seen in the in vitro sections. The size of the trocar-hole should also be compared here to the microphoto of the trocar of the clamp (Fig. 18). On the sections from the in vivo experiment and in the close proximity of the trocar-hole, we find the same mechanical disturbances of the bone tissue as was found in vitro: crushed bone, microcracks producing microfragments, and irreversibly deformed osteons (Fig. 29). This damage must disturb the vascularity in this region. The same onesided distribution of this damage is found here, confiig that it is probably caused by the preloading stress on the bone from the pinless clamp. As distinct from the in vitro histology findings, the in vivo sections additionally demonstrate the reactions of the living bone to mechanical and vascular disturbance of the tissue, such as callus formation, resorption and remodelling.

Fig. 26a1 X-ray taken 3 weeks post-op. No reactions can be found. Fig. 26b: X-ray taken 4 weeks post-op. Callus can be seen to have formed. Fig. 26c: X-ray taken 4 weeks post-op. A small radiolucent zone due to resorption can be seen around the tip of the pinless clamp.

Fig. 28: The trocar-hole has the shape of a triangle which has been cut into the bone cortex by the trocar. The size should be compared with the size of the trocar (Fig. 18). Note also the callus formation. Sheep 019, SO posterior, magnification x 16, micmradiogmph.

Fig. 3Ob: The intact surface of the trocar-hole was measured and related to the possible length of the surface. Fig. 3Oc: The depth of the cortex remodelling was found was measured.

thickness

in

which

Fig. 3Odz The “sequestrum most likely to happen” was created on the drawings of the sections.

Cortex thickness

Fig. 29: The mechanical damage (microcracks, micmfragments and irreversible deformation) can be seen on the left side of the hole. Resorption and remodelling are very distinct in this section. Sheep 019, S-l post., magnification x 16, fuchsin

staining. Drawings were made of the sections at a magnification of 50x. On these drawings, different measurements were made to quantify the extent of these reactions (Fig. 30 a-d).

Fig. 31: Cortical thickness measured on the SO sections through the central part of the trocar-hole at a right angle to the cortex surface. Pinhole S~nhole

depth

Depth of Pinless Clips (mm)

Ph rile so Fig. 30a: The diagramme shows how the cortical thickness and the depth of the trocar-hole were measured.

Fig. 32: Depths of trocar-holes of pinless clamp measured on 01; So sections at the deepest po’mt at a riglk angle to the cortex surface.

Moe Stene: Biomechanikalevaluation

s 21

Trocar-holes of almost equal depths were created (ca. 1.2 mm), but the cortical thickness of the tibia varied for the different sheep. This means that the trocar-hole may look deeper on some of the sections, see Figs 31 and 32, and compare Fig. 39 with 41. This also means that the application technique of the clamp is reproducible. Reactions of living bone tissue: 1. Callus: External callus is seen to have formed on the surface of the cortex around the trocar-hole (Fig. 28). 2. Resorption: Fig. 33 shows an insertion site in the cortex in trocar-hole at greater proximity of the the close magnification. On the edges of the triangular trocar-hole a groove is visible which has been formed by the resorption of cortical bone around the trocar tip (see also Fig. 29). This resorption groove has worked its way down along the intact bone surface of the trocar-hole.

Fig. 33: The resorption groove on the edges of the trocar-hole can be seen. Sheep 059, SO post., magnification x 50, fuchsin staining.

3. Remodelllngz In the cortex around the trocar-hole, increased bone activity can be seen especially in some of the sections. Primarily remodelling is seen, but also broadening of natural canals can be found (Have&an, Volkmanns and other natural canals for vascularity). Resorption spaces can be seen in the proximity of the trocar-hole indicating that a resorption process has been going on around the insertion site starting from the edge of the trocar-hole. In some of these spaces resorption is still going on, but in most of them new bone formation has begun. This can be seen especially in the fluorescence pictures and the microradiographs (Figs 35 and 36). From the fluorecence microscopy, it can be seen that this new bony apposition has mainly been built up from the third week onwards because green colour from the calcein green is visible almost exclusively.

Fig. 35: Increased amount of mmodelling activity around the trocar-hole can be seen. Sheep 019, SO anterior, magnification x 30, microradiography.

Fig. 34 compares indirectly how much of the intact bone surface in the different trocar-holes has been resorbed and how much is still intact As can be seen in the diagramme, on average more than 50% of the trocar-hole surfaces are still intact. The bone surface in the deeper part of the hole must at this time provide a secure hold for the trocar tips of the pinless clamp. However, this bone tissue is mechanically disturbed to some extent (Fig. 29). Intact bone surface in pinhole as % of maximum possible surface

Fig. 36: It can be seen here that in most resorption spaces a bone formation process has begun. Fluorescence microscopy. Pill rite =

Sh..p

019

m

Sheep 059

ShWP

-as

Fig. 34: The surface of the still intact trocar-hole surface measured as a percent of the maximum surface length. This diagramme shows the part of the trocar-hole surface, which has not been removed by resorption after 5 weeks of in vivo application and with functional loading of the clamp.

In some sections remodelling is actually seen through the whole cortex underneath the trocar-hole (Fig. 35 and 39). To be able to quantify the reactions, one has to look at the neighbouring sections as well (compare Figs 28, 27 and 40). A comparison between the extent of remodelling in the cortex around the trocar-hole in the three sections at each insertion site is shown in Fig. 37. As can be seen from these diagrammes, there is a range of remodelling activity halfway through the cortex, with somewhat more at the posterior insertion sites.

S22

removal. Both the extent and the distribution of the remodelling activity can be seen to be very different. At one insertion site, we see intensive activity just near the trocarhole, whereas in others minor activity is spread throughout the cortex, and again others show no increase in activity. Especially at the posterior insertion sites intensive reactions are seen (compare Figs 29 and 39).

Percent of Cortical Thickness with Remodelling

Pin rile + Sh..p

A Sheep OS9

019

l

sIl..p

-1S

Fig. 37: The diagramme shows how deeply in the cortex under the trocar-hole remodelling can be found. The depth is measured from the cortex surface. Note that the diagramme does not quantify the rernodelling process. The resorption and remodelling process carries the risk of a sequestrum forming around the trocar-hole (Fig. 29). The possibility of sequestmm formation has been quantified for each insertion site by measuring the sequestrum circumference most likely tb occur (the resorption spaces around the hole were connected at a distance under the trocar point so that if a sequestration arose, the amount of clamping force would decrease substantially) (Fig. 38). As this diagramme shows, for all the insertion sites (calculated as a mean value between the three sections), the length of intact bone in terms of the circumference of the sequestrum most likely to develop is 50% or more of the circumference.

Fig. 39: An extensive resorption groove can be seen, and a deep remodelling process. Sheep -28, SO post., magnification x 30, micmradiograph.

Length of Intact Bone as % of possible sequestrum length % LOO

j:l; 1 : / : , : i l&ax

S-I

so

s+, Anterior:

so

S-l

WI

Pin 6110 l

Sh.epOI9

A Sheep069

l

Shwp -18

Fig. 40: An insertion site with minimal resorption and very localised remodelling. Note this is the neighbouring section to Fig. 29. Sheep 019, SO post, magnification x 50, microradiography.

Fig. 38: The circumference of the sequestrum most likely to form was measured (a sequestrum, which could form underneath the trocar point). Here the length of intact bone as a percentage of this distance is shown. Quantitative

evaluation:

1. Between the 6 trocar-holes

of the pinless clamp

This pattern of events in the bone as a qualitative reaction to the trocar of the pinless clamp is more or less the same at all insertion sites. The extent and distribution of the bone reactions: the resorption groove and the remodelling zone around the insertion sites varied considerably between the three sheep and between the posterior and the anterior insertion sites. This can be seen in Figs 34, 37 and 38, in which the different bone reactions at the six insertion sites are compared, as well as on the photomicrographs (Figs 3941). At some of the insertion sites, we see an extensive resorption groove, where almost the whole side surface of the trocar has gone, in others we only see minimal bone

Fig. 41: Insertion site with very little resorption groove and almost no remodelling to be seen. Sheep 059, SO ant, magnification x 30, fuchsin staining.

Moe Stene: Biomechankal evaluation

2. With a Schanz screw trocar-hole If we compare the size of a trocar-hole of a pinless clamp with the surrounding disturbed tissue to the size of a trocarhole from a classic Schanz screw, even without surrounding disturbance, it can be seen that the area of the trocar-hole of a pinless clamp is much less than the trocar-hole size of a normal external fixator pin (Pig. 42). From this comparison, it can be said that the increased activity around the trocar-hole of the pinless clamp, which disturbs the bone tissue (e.g. resorption, microfractures, irreversible deformation), is seen as a local reaction directly around the trocar-hole. Since the trocar-holes themselves are small, the amount of destruction of the bone is limited.

S23

The discussion will be divided into mechanical aspects. 4.6.1 Mechanical

a)

Since calibration after removal of the strain gauges showed a return to zero after they were taken off after the 5 weeks of experimentation, the drift of these devices cannot have had any influence.

b)

Creep or deformation of the clamp material. The dynamic mechanical test revealed that a plastic deformation can explain less than 0.1 mm deformation of the clamp arms (both of them together), which accounts for a clamping force decrease of less than 10 N. This explains part of the clamping force decrease which was found in vivo.

4

Trocar penetration because of movement (self-drilling). This did not happen in the dynamic in vitro experiment and therefore it is questionable whether we can expect it to happen in vivo.

4

Stress relaxation and time dependent properties of the bone. Stress relaxation and time dependent properties are known to play a role for the fit few days when one applies load, e.g. compression to bone [75]. Hence, this can only explain an initial decrease of the clamping force. Since we do not see any substantial decrease in the first few days, it cannot have any major influence on the clamping force of the clamp. It should also be remarked that we did not see any decrease of pmload in the in vitro experiments which could be explained by these bone properties, although this has been found in other experiments [75, 85).

4

Resorption and remodelling. A possible explanation of force relaxation may be that the bone becoming pomtic becomes more deformable. This theory does not seem to explain the major loss of force (28%) as the cortical thickness of bone concerned (half of the thickness of the cortex assumed) is about 2 mm. A change from dense bone to 30% porosis would reduce the Young’s modulus from 2000 kp/mmz to say 1300 kp/mm% Furthermore, the change in distance between the tips of the clamp to produce a 28% change in force would need to be in the order of 0.4 mm. These figures do not fit for a sole explanation of the reduction of force due to the bone becoming pomus.

4.6 Discussion

The main question to be evaluated was whether the Pinless external fiator would keep its holding power during these 5 weeks and to determine the biological reactions at the trocarto-bone interface. The mechanical aspects were evaluated in two mechanical tests dealing with the question of stress relaxation or plastic deformation in the clamp arms; in two in vitro experiments, questions concerning viscoelastic properties of the bone and self-drilling activity due to movement of the clamp were investigated, in vivo the clamping force was determined. The biological aspects were evaluated by in vitro and in vivo histology.

aspects

Strain measurements of the clamp applied in vivo revealed a decrease of clamping force with a mean value of 28% (25 N). The possible reasons for this decrease in clamping force will be explained:

Fig. 42: Comparison between a 4.5 mm Schanz screw trocarhole and a trocar-hole of a pinless clamp (clamp moved back and forth 10 times for application). In vitro specimen, magnification x 18.

A newly constructed device: The Pinless external fixator has been evaluated mechanically and in vivo. A model was used in which the loading of the clamp could be exactly controlled. The clamp was applied to sheep tibiae and functionally loaded (50 N, 150 cycles/day) for 5 weeks. The loads and time range of the experiment were chosen mainly because we are discussing the indications for the Pinless external fixator as a temporary fracture stabilisation device. This means we want to simulate the loading situation occurring in a polytraumatised patient who is lying in bed and not bearing weight One possible kind of loading on the fixator could be that in which the leg has to be lifted up or supported in a sling. The load of the leg, e.g. 200 N, would then be distributed to at least 4 clamps, which results in a load on each clamp of 50 N. Furthermore, we know from the literature that bone reactions appear approximately 3 weeks post-op 12, 481.

and biological

4.6.2 Biological aspects No clinical signs of infection were found amund any insertion site during the 5 weeks of experimentation On two swabs sent for bacteriological testing, growth of staphylococcus epidermis was found. Since no signs of inflammation or bacteria could be found using the microscope, and since staphylococcus epidermis belongs to the normal skin flora, the reason for the positive tests is most probably contamination from the skin at the time of taking the swabs. The two Positives will therefore not be considered pathological.

S24

Histology cannot explain the clamping force decrease. We found Haversian remodelling of various intensity in the surroundings and especially underneath the trocar point. Haversian remodelling is able to change the elasticity of the bone, but as already discussed the preloadmg force is too small to be regarded as significant. Another possible reason for this decrease, apart from deformation of the material, could be movement of the trocar tips in the living bone of approximately 0.1 mm on each side, although these selfdrilling properties were not found in the in vitro experiments.

work at all as we had intended, this explains the small number of experimental animals used. However, the results from the three sheep are consistent and seem to support the view that the Pinless external fiiator will offer fiiation with minimal disturbance to the bone tissue.

At removal the pinless clamp was observed to have maintained a secure hold on the bone. This was confirmed by the clamping force measurements (the clamping force was still 72%). Using the scanning electron microscope, we saw that the trocar-hole of the pinless clamp is round immediately after application. Therefore, from the start we cannot have had a fit over a large surface of the tip, since we know that the trocar point has sharp edges. It is to be expected though that we have contact at the edges. We learned from histology that we only have an approximately 58 % “intact” surface of the trocar-hole and that this is a surface of partly disturbed bone (microcracks, irreversible deformation). A smaller area of contact increases the risk of local overload. As the area of contact increases as a result of local&d fragmentation the situation becomes “stable”. This situation in the trocar-hole must be sufficient to secure the clamp. Since the pinless clamp sustains the preload it must still have some substance to clamp around, which seems to be of major importance for maintaining the clamping and holding force of the clamp.

Mechanically, the pinless clamp sustained its holding power very well, all three clamps used in vivo were clinically tight at removal and we saw no signs of slippage. Measurements of the clamping force revealed a decrease of only 28%. This decrease cannot be attributed to changes of the Young’s modulus of bone due to increased porosity. Plastic deformation of the clamp arms explains less than 10 N of the decrease. The decrease was mainly observed during the fit 20 days, then the clamping force tended to stay at a constant level. This means that after 5 weeks of functional loading of the clamp, we have 72% of the initially applied clamping force.

The resorption groove seen on the edge of the trocar-hole of the pinless clamp is probably due to micromotion between the implant and the bone since it has exactly the same appearance as the resorption grooves around the Schanz screws due to micromotion [48]; moreover, the mechanical disturbances from the application of the clamp (microcracks etc.) must also have disturbed the bone tissue in this area and thus led to bone resorption. On the sections from the in vitro experiment, the mechanical disturbances are mainly visible in the area of the resorption groove seen in the sections from the in vivo experiment. Furthermore, part of the reason for the increased remodelling is probably due to biological tissue disturbance caused by application of the clamp as this damages the vascular&y in the region. Whether part of the remodelling process is caused by the pressure of the trocar points or by vascular disturbance from the pressure cannot be concluded, although pressure alone is improbable. The reaction of living bone to the pinless clamp is qualitatively the same as has previously been established for Schanz screws. [48]. If we were to compare the extent of bone disturbance made by a pinless clamp and that made by a Schanz screw, we would find that the area of the trocarhole of a pinless clamp, which we know to be very small, plus the surrounding area of disturbed bone tissue, is nonetheless considerably smaller than the area of the Schanz screw hole alone. The amount of bone tissue affected differs correspondingly, furthermore, the area around the Schanz screw also demonstrates resorption processes and reactive osteoporosis. In effect, the amount of bone tissue removed by pinless external fixation is comparably negligible. This study was planned as a pilot experiment which would show whether the concept of pinless external fixation would

47 conclusions The Pinless

external fiiator as constructed over existing systems of external fixation.

has advantages

Biologically, we had no signs of infection around insertion sites and the histological investigations showed very local&d disturbances of the bone tissue, the trocar-hole from the pinless clamp being 1.2 mm deep. This study therefore concludes that it is safe to use the Pinless external fixator for temporary fracture stabilisation. Acknowledgementt~ We would like to thank the team of the AO/ASIF Research Institute in Davos for their help and support In particular, we would like to express our gratitude to Prof. Dr. med. B.A. Rahn, C. Scandella, P. Dlscher, P. Rijmer, C. Sigrist, C. Gmnvillano, E. Rampoldi, J.-P. Imken, C.Giintensperger, and E.Omerbegovic. Our very special thanks go to Prof.Dr.med. S.M. Perren, who supervised this research. 4.0

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Moe Stene: Biomechanikal evaluation

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