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To evaluate if redirecting a Kirschner wire (K-wire) through the same proximal hole will weaken the pull-out force and to test if multiple redirections will result in a continued stepwise decrease in pull-out force.
An Instron was used to test the pull-out force of K-wires using the peak initial failure load as a measure of failure of K-wire fixation. K-wires 0.062 inches in diameter were inserted with an angled drill guide into a bicortical bone substrate. Trials were divided into 7 groups with the first group having the K-wires placed through both cortices and then tested without redirection. In groups 2–6, the K-wire was placed bicortically and then withdrawn and redirected through the same proximal hole with 1, 2, 3, 4, and 5 redirections. A control group in which the K-wire was only unicortical was also tested.
Compared with the control group of no redirects, any number of redirections weakened the pull-out force. There was no difference between redirected groups and the unicortical group. When comparing between redirections, there were no significant differences in pull-out force. Regression analysis showed that, after the first redirection, there was no stepwise change in pull-out force with additional redirection.
There was a significant decrease in pull-out force with any redirections, but there was no stepwise decrease in failure force after multiple redirections. The failure force of any redirection was similar to a unicortically placed wire.
Any K-wire redirection attempts in hand bone fixation can result in a considerably weakened construct.
The benefits of using these smooth wires in the context of hand surgery include percutaneous placement, simple removal in an outpatient setting, and low cost. Extra-articular K-wires have the same recovery and functional results as lag screws and plates in certain cases and may be preferred to nonsurgical treatment with cast or brace immobilization.
Smooth wires biomechanically rely on friction for fixation, though these have evolved over time from thick 3.5- to 6-mm pins that were hammered into predrilled holes to the drilled 0.7- to 1.5-mm wires that are typically used for fracture fixation today.
However, none of these studies look at the biomechanical limitation of the smooth wires themselves, as the lack of additional features such as threads could be a concern in settings of poor bone quality or bone loss. In addition, K-wires require accurate placement and failure to achieve that may lead to either redirecting the K-wire through the same proximal hole or picking a new starting hole. Redirecting the K-wire may weaken the overall construct by loosening the friction-based fixation at the proximal insertion point and potentially lead to failure. Therefore, we aimed to understand the effect of multiple redirection attempts on K-wire pull-out force.
We hypothesized that a smooth wire redirected through the same proximal hole will weaken the pull-out force. We also hypothesized that the pull-out force will continue to decrease with each redirection and eventually reach the failure force of a unicortically placed K-wire.
Materials and Methods
We elected to use 0.062-inch-diameter K-wires as our smooth wire and a bicortical foam plate (Sawbones, Vashon Island, WA) as our biomechanical bone analog, which consisted of two 3-mm slabs of cortical foam (40 pounds per cubic foot/640.7 kg per cubic meter) on the outside with 40 mm of trabecular foam (20 pounds per cubic foot/320.4 kg per cubic meter) in between. An Instron testing machine (Norwood, MA) was used to test the frictional pull-out force of the K-wires (Fig. 1A) and the pin was always pulled in line with the plane of insertion, reorienting the block as necessary. The peak initial failure load in Newton was the outcome of interest.
Using a custom angled guide, the K-wires were directed 10° from perpendicular to the foam plate substrate before drilling to maintain a uniform substrate thickness (Fig. 1B). Trials were divided into 7 groups with 10 samples per group. A sample size estimate for analysis of variance testing for 7 groups showed that a sample size of 10 would result in 87% power to find a difference with an effect size of 0.5 N with 95% confidence. In the first group, the K-wires were placed through both cortices and then tested without redirection. In the second group, the K-wires were placed through both cortices followed by withdrawal of the K-wire from the distal cortex and redirection through the same proximal hole into a new distal cortex location. All drilling, including redirections, were performed with the 10° guide, rotating around the proximal cortex starting hole, to maintain a similar amount of foam substrate though all drill attempts. Groups 3–6 represented 2, 3, 4, and 5 redirections, respectively. In group 7, the K-wire was only placed into the proximal cortex and drilling was stopped before any engagement into the distal cortex to simulate the effects of placing a single unicortical wire.
After data collection, analysis of variance testing with post hoc Tukey tests was performed. A linear regression analysis was used to assess how much, if any, consecutive redirections of the K-wire using the same proximal hole would cause a reduction in fixation strength.
As measured against the control group of no redirects (72.7 ± 19.6 N), redirecting 1 or more times weakened friction fit as measured by pull-out force (1 redirect: 35.2 ± 13.0 N; 2 redirects: 37.2 ± 14.4 N; 3 redirects: 32.2 ± 10.0 N; 4 redirects: 40.6 ± 12.0 N; 5 redirects: 37.9 ± 8.0 N; all P < .05 against control). There were no differences between the pull-out force of any redirected groups when compared with the unicortical group (40.4 ± 7.4 N; all P > .05).
When comparing the pull-out force of 1 or more redirects, there were no significant differences in the friction fit between all groups (all P > .05).
Regression analysis showed that after the first redirect, there was no significant stepwise change in failure force with additional redirection (R2 = 0.01, P = .45) (Fig. 2). When the control group was included in the regression as the “0 redirect,” then redirection was found to be a significant factor that explained 18% of the variation in the strength (R2 = 0.18, P < .05).
K-wires are common implants in a hand surgeon’s armamentarium. Previous research has established many of the strengths and weaknesses of K-wire design and placement, such as thickness of the wire, shape of the wire tip, and method of insertion,
One unknown factor, however, is the impact of multiple redirections when placing a K-wire. Because of percutaneous placement of K-wires, it can be difficult to place them in the ideal distal position. The surgeon must then redirect the K-wire, often using the initial proximal hole. Although there are a variety of reasons for poor clinical outcomes after K-wire fixation, pull-out failure force was used for this study because it is often used to assess the efficacy of a construct in a biomechanical model and the measurement of pull-out force would give the most accurate representation of the change in friction force, though this may not be the sole limitation in a clinical model.
Thus, factors that decrease bone strength, such as the fracture pattern, bone quality, or surgeon variables, may lead to loss of fixation in the absence of additional methods for compression. We sought to evaluate the potential for loss of strength from redirecting K-wires through the same proximal hole in a bone plate model, because widening of the proximal hole was hypothesized to have the effect of providing only unicortical fixation with the distal hole.
In our study, we found that after only 1 redirection attempt, there is a statistically significant decrease in pull-out force compared with control. This was true for any number of redirections. Although the literature shows that unicortical and bicortical fixation for locking plates have similar failure forces,
the biomechanical fixation of K-wires is very different compared with locking screws. Biomechanical studies have shown K-wires to have similar outcomes to locking plates in the small bones of the hand, but literature on an analogous unicortical versus bicortical K-wire construct is lacking.
showed that redirecting pedicle screws led to a 24% decrease in failure force.
We also found that the loss of pull-out force did not increase with additional redirections. There was also no difference in force between a unicortical K-wire and a redirected wire, indicating that even a single redirection weakens the proximal cortical fixation to the point where the wire relies solely on the distal cortex for its fixation. There are no known studies that compare unicortical wires with bicortical wires in vivo or that comment on how much pull-out strength is adequate for construct strength in a clinical context. In the case of osteoporotic bone or other deficiencies in bone strength, unicortical fixation may not be adequate.
There are several limitations of our study. We recognize that K-wire pullout is not the usual mode of failure and that fracture stability after K-wiring also depends on multiple factors including fracture geometry, interdigitation, and orientation. Ultimately, K-wire stability depends on cortical purchase, leading to our choice to investigate the friction force of the K-wire. In addition, it is unknown what the clinically acceptable pull-out force is for K-wire fixation of a fracture, making unclear the clinical effect of the differences found by this study. We believe that our study provides the clinician with knowledge to make a more informed judgment during a surgical case because it is important to recognize that the redirection of a wire may result in a nearly 50% loss of pull-out force. Bending, axial loading, and torque metrics for the bone substrates should be studied to further compare the impact of redirections. It would also be beneficial to observe the frequency of redirections occurring in a clinical setting and to follow patient outcomes.
Another limitation of our study was the use of sawbones as a fixation substrate. We selected this model to eliminate as many variables as possible, such as quality of bone, thickness of cortical and cancellous bone, and any periosteum or soft tissue attachments. The use of a flat plate allowed us to redirect while maintaining the same substrate thickness through all redirections. However, periosteum and other attachments may contribute substantially to fixation stability and additionally the remodeling of bone may play a role in the long term.
Although the clinical difference is uncertain, it may be advisable to avoid any redirections of a K-wire through the same proximal hole, especially for fixation in a case that demands optimal K-wire placement, or in a patient with uncertain bone quality.