Veterinary Surgery 27:301-306, 1998

Holding Power of Different Pin Designs and Pin Insertion Methods in Avian Cortical Bone

LAUREL A. DEGERNES, DVM, Diplomate ABVP, SIMON c. ROE, BVSC, PhD, Diplomate ACVS, and C. FRANK ABRAMS, Jr, P ~ D , PE

Objective-To measure pullout strength of four pin types in avian humeri and tibiotarsi bones and to compare slow-speed power and hand insertion methods. Study Design-Axial pin extraction was measured in vitro in avian bones. Animal Population-Four cadaver red-tailed hawks and 12 live red-tailed hawks. Methods-The pullout strength of four fixator pin designs was measured: smooth, negative profile threaded pins engaging one or two cortices and positive profile threaded pins. Part 1: Pins were placed in humeri and tibiotarsi after soft tissue removal. Part 2: Pins were placed in tibiotarsi in anesthetized hawks using slow-speed power or hand insertion. Results-All threaded pins, regardless of pin design, had greater pullout strength than smooth pins in all parts of the study (P < .OOOl). The cortices of tibiotarsi were thicker than the cortices of humeri ( P < .OOOl). There were few differences in pin pullout strengths between threaded pin types within or between bone groups. There were no differences between the pullout strength of pins placed by slow-speed power or by hand. Conclusions-There is little advantage of one threaded pin type over another in avian humeri and tibiotarsi using currently available pin designs. There were few differences in pin pullout strengths between humeri and tibiotarsi bones. It is possible that the ease of hand insertion in thin cortices minimizes the potential for wobbling and therefore minimizes the difference between slow-speed drill and hand insertion methods. Clinical Relevance-Threaded pins have superior bone holding strength in avian cortices and may be beneficial for use with external fixation devices in birds. OCopyright 1998 by The American College of Veterinary Surgeons

IRDS HAVE UNIQUE skeletal modifications B to reduce weight during flight, including loss or fusion of bones, reduction in cortical bone thickness, and pneumatization of medullary cavities.' The thin, brittle cortices, however, often complicate avian fracture repair because of poor pin or screw holding power and potential problems with iatrogenic frac- tures.2 Other complications of avian orthopedics in- clude small patient size, scant soft tissues covering distal extremities, and fractures near joints that often

lead to reduced joint m~bi l i ty .~ Most of the pioneer- ing work in avian orthopedics has been done in rap- tors using techniques adapted from small animal^.^.^ A few controlled studies of avian fracture healing and stabilization have been conducted, using pigeons as a m0de1.~-~

External skeletal fixators (ESF) have been suc- cessfully used in a variety of avian fracture^.^.^.'^." Premature pin loosening at the fixator pin-bone in- terface, however, is one of the most common

From the Department of Companion Animals and Special Species Medicine, College of Veterinary Medicine, and the Department of Biological and Agricultural Engineering, College of Agriculture and Life Sciences, North Carolina State University, Raleigh, NC .

This study was supported by a Department of Companion Animals and Special Species Medicine Grant, College of Veterinary Medicine, North Carolina State University.

Address reprint requests to Laurel Degernes, DVM, Department of Companion Animals and Special Species Medicine, College of Veterinary Medicine, NCSU, Raleigh, NC 27606.

OCopyright 1998 by The American College of Veterinary Surgeons 016 1 -3499/98/2704-0002$3.00/0

301

302 HOLDING POWER OF PINS IN AVIAN CORTICAL BONE

causes of ESF failure in birds2 and Biomechanical studies in mammals have investi- gated the influence of pin design and pin insertion methods on the bone-holding power of pins. Smooth pins have less resistance to pullout com- pared with end-threaded pins with negative profile threads (threads cut into the shaft).I4-l6 End- threaded pins with double cortical contact provide better holding power than single cortical contact end-threaded pins.I4 Pins with a positive thread profile (outer thread diameter greater than shaft diameter) have been shown to have increased strength and stiffness and superior bone-holding power.'6317 Other elements of the pin design, such as the diameter of the pin and the tooth profile and pitch (number of threads per unit length) of the threaded portion, also influence the holding power."

The method of pin insertion has also been identi- fied as a major influence on premature pin loosening in dog^.'^,'^ High-speed power drilling or slow-speed power drilling without predrilling the hole can cause thermal necrosis and osteocyte damage. 12~19*20 Hand insertion (eg, Jacobs chuck) may cause more struc- tural damage at the pin-bone interface because of wobbling during pin insertion. '83'9 Slow-speed power drilling or predrilling with a twist bit, followed by hand insertion, has been shown to result in greater holding power as measured by the axial force re- quired to extract the pin from the bone.I4.l9

The objective of this study was to measure pullout resistance of four pin types in avian humeri and tibiotarsi bones and to compare slow-speed power and hand insertion methods. Commercially available orthopedic pins were designed for mammals, so it was not known if these pins would have similar biomechanical properties in avian cortices. We hy- pothesized that the pin type, bone type, and method of pin insertion would affect the resistance to pin pullout from avian cortical bone. This study was divided into two separate parts. The objective of the first part of the study was to measure and compare the resistance to pin pullout of four different fixator pin types in the humerus (pneumatic) and tibiotarsus (medullary) bones. The four pin types were bicortical smooth pins, bicortical pins with negative profile threads engaging one (NEG- 1) or two (NEG-2) corti- ces, and bicortical pins with positive profile threads (POS). The objective of the second part of the study was to measure and compare the resistance to pin

pullout of each of the four pin types after pin inser- tion into the tibiotarsal bone using slow-speed power (150 rpm) or hand insertion.

MATERIALS AND METHODS

Four frozen red-tailed hawk (Buteo jamaicensis) car- casses and 12 permanently disabled but otherwise healthy red-tailed hawks were obtained from a raptor rehabilita- tion center. All of the birds were older than 1 year of age as determined by plumage. Red-tailed hawks were selected because of their large size (900 to 1,200 g) and availability from wildlife rehabilitation centers. These hawks are suitable as a model for avian orthopedic re- search because they are representative of birds commonly admitted to wildlife rehabilitation centers with orthopedic injuries. The live hawks were housed in a large flight pen and were fed thawed laboratory rodents once daily and provided fresh water ad libitum. None of the birds had humerus or tibiotarsus fractures, as determined by physi- cal examination before the study and gross examination of the bones after pin placement and soft tissue removal. This study was approved by the North Carolina State University Institutional Animal Care and Use Committee.

Part I : Pin Pullout Tests on the Humerus and Tibiotursus Bones

Eight humeri (group 1) and tibiotarsi (group 2) were dissected from the thawed hawk carcasses, and all soft tissues were removed. The bones were wrapped in saline- soaked gauze sponges and frozen at -20°C until used in the study. Commercially available stainless steel pins with trochar points (IMEX Veterinary, Inc, Longview, TX) were used in the study (1 2 of each pin type per group): (1) smooth intramedullary pin; (2) partial end-threaded, negative profile intramedullary pin with single cortical contact (NEG- 1); ( 3 ) partial end-threaded, negative pro- file intramedullary pin with double cortical contact (NEG- 2); and (4) partial end-threaded, positive profile acrylic half-pin (POS; 1.75 mm outside thread diameter). All of the pins were new and were used only once during the study. All of the pins had the same shank diameter (1.6 mm), and all of the threaded pins had the same thread pitch (0.55 mm or 1.8 threaddmm). Ellis pins (Kirschner Medical Corporation, Timonium, MD) were not available in the size required for these birds, so the partial end- threaded, negative profile pins were placed with either single cortical contact (with approximately 1.5 cm threaded pin exposed on the far cortex to allow for thread contact with the far cortex only) or double cortical contact (with threads engaging both cortices). Six pins were placed at 1.5-cm intervals along the diaphysis of each

DEGERNES. ROE. AND ABRAMS 303

bone. The pins were placed perpendicular to the long axis of the bone, from medial to lateral in the tibiotarsus and lateral to medial in the humerus. Two pins of each pin type were placed at each of the six locations, to account for possible differences in the cortices at different sections of the bones. All pins were inserted with a slow-speed power drill (< 150 rpm) until the trochar point extended completely through the far cortex. Before placement of the POS pins, the pin site was predrilled with a smooth 1.6-mm trochar point pin.

The bones were placed in a specially designed holding device during pin extraction. Pins were connected to the load cell by a drill chuck. The maximal force required to pull the pin from the bone was measured using a universal testing machine (Instron Model 1122; Instron Corp, Can- ton, MA), at an extraction rate of 100 mdmin. The peak load for each pin extraction was recorded from the strip chart. After pin removal, cross-sections of the bones were cut on a band saw next to each pin insertion site (within 2 mm). Cortical thickness was measured with calipers at four locations next to the pin insertion site, on the near and far cortices, and averaged.

Part 2: Pin Pullout Tests on Tibiotarsus Bones Using Slow-Speed Power Drill and Hand Insertion

Twelve live, anesthetized hawks were used to simulate a clinical surgical procedure in which muscles obscure the approach to the bone and make bone stabilization during pin insertion more difficult. The hawks were anes- thetized with an intramuscular injection of ketamine (10 mgkg) and xylazine (2 mgkg). The feathers over each tibiotarsus were plucked from the stifle to the tarsal joint. The 12 birds were divided into two groups: Group 1 had pins inserted using a slow-speed power drill (150 rpm), and group 2 had pins inserted by hand using a Jacobs chuck without a pin guard. Pins of each of the four types were placed in the tibiotarsus from medial to lateral. Three pins of each type were placed at each of six pin locations (six pins per bone at 1.5-cm intervals for a total of 18 pins of each type). The pin sites were predrilled with a smooth 1.6-mm trochar point pin before placement of the POS pins. The hawks were euthanatized immediately after pin placement. The soft tissues were dissected off the tibiotarsi, and the bones were frozen in saline-soaked sponges until testing. The bones were thawed for 2 to 4 hours before testing. Testing of pullout forces and mea- surement of cortical thickness were done as described in Part 1. Cortical thickness data at each pin site were com- bined from the eight tibiotarsi in Part 1 and 24 tibiotarsi in Part 2.

STATISTICAL ANALYSIS A separate analysis of variance was performed for

each bone type in Part 1 or pin insertion method in

Part 2 to determine differences between pin types within each group. Pin pullout data were transformed (square root of the pullout variable) to equalize vari- ance between groups (PROC GLM, SAS 6.09). The pin type, bone side (left or right), and cortical thick- ness (average of four cortical measurements at each pin location) were included in the model to adjust for variation resulting from them. A pin type-cortical thickness interaction term was included in the origi- nal model to determine if changes in the pullout strength with cortical thickness were the same for all pin types. Because significant differences were not found in this interaction (P 2 .22), the final analysis did not contain this term. To determine which pin types were different, pairwise t-tests were carried out on the least square means of the pullout data. t-Tests were used to compare pin pullout data within each pin type between the humerus and tibio- tarsus groups and between the slow-speed drill and hand drill pin insertion groups (PROC t-test of SAS). Pairwise t-tests were done on the least square means of the cortical thickness to determine differences in cortical thicknesses at the six pin locations. The overall significance was set at P I .05. When multi- ple comparisons were made between the six pin loca- tions, a P value s .0167 was used to reduce the chance of a type I1 error.

RESULTS

Part 1: Pin Pullout Tests on the Humerus and Tibiotarsus Bones

Bone side (left or right) did not account for any effects (P 2 .15), so the data were combined within groups. The mean cortical thickness was signifi- cantly greater in the tibiotarsus than the humerus (Fig 1; P 5 .0001). Mean cortical thickness at the six pin locations ranged from 0.55 to 0.81 mm in the humerus (overall mean, 0.70 mm) and from 0.88 to 1 .O mm in the tibiotarsus (overall mean, 0.95 mm). There was no correlation in any of the groups be- tween cortical thickness and pin pullout (humerus, R2 = .28; tibiotarsus, R2 = .06).

There was a significant difference in pin pullout in the humeri and tibiotarsi based on pin type (Fig 2). The smooth pins had a lower pin pullout than any of the threaded pins for both long bones (P I .0001 for all comparisons). Within the humerus group, the NEG-2 pins had a higher pin pullout than

304 HOLDING POWER OF PINS IN AVIAN CORTICAL BONE

4.2

1 4 6 6

Fig 1. Cortical thickness by pin location. Mean 2 SD corti- cal thickness (mm) of the avian humerus and tibiotarsus at six pin locations, spaced at 1.5-cm intervals from proximal (1) to distal (6). The tibiotarsus cortex was significantly thicker than the humerus cortex at each pin location (P 5

.0001 for all comparisons).

the other threaded pins ( P I .009 for all compari- sons). There was no difference between NEG-1 pins and the POS pins (P = .27). Threaded pins were two to three times more resistant to pullout from the humerus than smooth pins.

Within the tibiotarsus group, there were no differ-

Fig 2. Pin pullout resistance in the humerus and tibiotar- sus. Pullout resistance (Newtons, mean 2 SD) of four pin types (n = 12): smooth, negative profile with single cortical contact (NEG-l), negative profile with double cortical con- tact (NEG-2), and positive profile pins (POS). The smooth pins had significantly lower pin pullout resistance than the three threaded pins in both sets of bones (P 5 .0001 for all comparisons). The NEG-2 pin had greater pin pullout resistance than the NEG-1 or POS pins in the humerus (P 5 .009). The NEG-1 pin had greater pin pullout resistance in the tibiotarsus compared with the humerus (P 5 .02).

350 '0° 1

Smoom NEG-1 NEG-2 POS 7 ~-

pln T~~ &ow Speed Drill .Hand I n s e r I i t i ~ ~

Fig 3. Pin pullout resistance using slow-speed power drill or hand insertion methods. Pullout resistance (Newtons, mean 2 SD) of four pin types (n = 18) in the avian tibiotar- sus: smooth, negative profile with single cortical contact (NEG-l), negative profile with double cortical contact (NEG-2), and positive profile pins (POS). The smooth pins had significantly lower pin pullout resistance than the three threaded pins in both pin insertion groups (P 5 .0001 for all comparisons).

ences between the pin pullout forces for any of the threaded pins (Fig 2; P 2 .92 for all comparisons). Threaded pins were three times more resistant to pullout from the tibiotarsus than smooth pins.

The pin pullouts were compared between the hu- merus and the tibiotarsus to determine if there were differences between pneumatic versus medullary bones. When resistance to pullout for each pin type was compared between the two groups of bones, only the NEG- 1 pins had greater strength in the tibio- tarsus (Fig 2; P 5 .02).

Part 2: Pin Pullout Tests on Tibiotarsus Bones Using Slow-Speed Power Drill and Hand Insertion

In both groups, all of the smooth pins had less resistance to pullout than the threaded pins (Fig 3; P I .0001 for all comparisons). None of the threaded pins were different from each other within the hand insertion group. The only difference between threaded pin types in the slow-speed power drill group was that the NEG-2 pins had a significantly greater pullout strength than the POS pins (P I .006 for all comparisons). Threaded pins were three to four times more resistant to pullout than smooth pins using either pin insertion method. There were no

DEGERNES, ROE, AND ABRAMS 305

differences between the slow-speed power drill and hand insertion groups when comparing pin pullout strength within each pin type.

DISCUSSION

Threaded external fixator pins provided more re- sistance to pullout than smooth pins in avian bones. This finding was in agreement with studies con- ducted in marnmal~.’~-’~*’’ An unexpected finding of the study is that there was little difference between pins with different thread designs. Because pins with larger diameter should have greater bone-holding strength, we anticipated that the positive profile pins would resist pullout better than negative profile threaded pins.16 The positive profile pin could have caused damage to the bone at the pin entry site. Microscopic studies of the bone at the pin entry site have shown that the bone surface is damaged by the threads until the threads engage and begin to cut a tract.” Because avian cortices are thin, a larger proportion of the bone may be damaged by the pin entry before the threads are formed. Microfissures at the bone-pin interface could have developed, despite predrilling.’’ Although no visible cracks were noted in the bone adjacent to the pins, we did not evaluate the pin-bone interface microscopically or radio- graphically in this study.

End-threaded, negative profile pins with double cortical contact provide better holding power than similar pins with single cortical contact (eg, Ellis pins) in canine bone.14 Ellis pins were designed to minimize the potential for breakage at the weak threaded-nonthreaded portion of the pin because this weak section of the pin is protected within the med- ullary ~ a v i t y . ’ ~ , ~ ~ In our study, we observed greater bone-holding strength in the NEG-2 pins in the hu- merus compared with the NEG-1 pins.

Bone-holding strength of threaded pins increases linearly with increasing cortical bone thi~kness.’~ We expected to find differences in pin pullout strength between tibiotarsi and humeri because tibio- tarsi cortices were an average of 26% thicker than humeri cortices. The only difference we observed was in the NEG-1 pin group, which had greater pin pullout strength in the tibiotarsus when compared with the humerus. Because only half of the threads were engaged in cortical bone in the NEG-1 pin group, compared with the NEG-2 and POS pin groups, the contribution of each thread engaged in

bone could be important. When the number of en- gaged threads was decreased even more in the thin humerus cortex, the difference between threaded pin types became statistically significant. The differ- ences between the humerus and tibiotarsus in pin pullout strengths of the other pin types could have been masked by wide sample variation.

The method of pin insertion has been identified as a major influence on premature pin loosening in

Our study did not show any differences within each pin type between the two pin insertion methods. Little effort is required to drive pins manu- ally through thin avian cortices when compared with manual pin insertion in thicker cortices. Because of this, wobbling during pin insertion is less likely to occur. Proper technique must be used during pin insertion to avoid wobbling. We did not measure pin tip temperatures during pin insertion and did not evaluate bone-holding strength in a long-term study. It is possible that if thermal necrosis occurred during pin insertion, more differences would have been evi- dent at a later time.

Threaded pins, regardless of their design, have better resistance to pullout than smooth pins in avian long bones. There is reduced pin pullout strength when single cortical contact negative profile pins are placed in the avian humerus compared with the tibiotarsus. For all other pin types studied, however, the pin pullout resistance from the humerus is com- parable to that from the tibiotarsus despite the thicker cortex in the tibiotarsus. It is likely that the thin cortices of avian bone diminish the influence of dif- ferent threaded pin designs. The influence of pitch and thread depth on the pullout strength of implants in thin cortical bone deserves further study. There is no difference in the pullout strength of pins inserted in avian long bones by either slow-speed power or hand methods.

ACKNOWLEDGMENT

The authors thank Debbie Whitt Smith and Timothy Seaboch for their technical expertise and Judith Jayawick- rama for statistical support.

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