Surgical Treatment of Long-Bone Deformities: 3D Preoperative Planning and Patient-Specific Instrumentation
Congenital or posttraumatic bone deformity may lead to reduced range of motion, joint instability, pain, and osteoarthritis. The conventional joint-preserving therapy for such deformities is corrective osteotomy—the anatomical reduction or realignment of bones with fixation. In this procedure, the bone is cut and its fragments are correctly realigned and stabilized with an implant to secure their position during bone healing. Corrective osteotomy is an elective procedure scheduled in advance, providing sufficient time for careful diagnosis and operation planning. Accordingly, computer-based methods have become very popular for its preoperative planning. These methods can improve precision not only by enabling the surgeon to quantify deformities and to simulate the intervention preoperatively in three dimensions, but also by generating a surgical plan of the required correction. However, generation of complex surgical plans is still a major challenge, requiring sophisticated techniques and profound clinical expertise. In addition to preoperative planning, computer-based approaches can also be used to support surgeons during the course of interventions. In particular, since recent advances in additive manufacturing technology have enabled cost-effective production of patient- and intervention-specific osteotomy instruments, customized interventions can thus be planned for and performed using such instruments. In this chapter, state of the art and future perspectives of computer-assisted deformity-correction surgery of the upper and lower extremities are presented. We elaborate on the benefits and pitfalls of different approaches based on our own experience in treating over 150 patients with three-dimensional preoperative planning and patient-specific instrumentation.
Bone and joint disorders are a leading cause of physical disability worldwide and account for 50 % of all chronical diseases in people over 50 years of age . Among these, the degeneration of cartilage (i.e., osteoarthritis) is the most common disease , affecting 10 % of men and 18 % of women aged above 60 years . The population is ageing thanks to increased life expectancy; accordingly, osteoarthritis is anticipated to become the fourth leading cause of disability by 2020 . One reason of early arthritis development is abnormal joint loading induced by bone deformity [4, 5]; i.e., the shape of the bone is anatomically malformed. Reasons for bone deformity may be congenital, caused by a birth defect, or posttraumatic if the bone fragments heal in an anatomically incorrect configuration after a fracture (i.e., malunion). Besides the development of osteoarthritis, bone deformity may result in severe pain, loss of function, and aesthetic problems . Particularly, gross deformities can cause quasi-impingement of the bones or increased tension in ligaments, resulting in a limited range of the motion. An unrestricted function is of fundamental importance for performing daily activities such as eating, drinking, personal hygiene, and job-related tasks. If not treated appropriately, salvage procedures such as arthroplasty (i.e., restoring the integrity and function of a joint such as by resurfacing the bones or a prosthesis) may be required.
Surgical treatment by corrective osteotomy has become the benchmark procedure for correcting severe deformities of long bones [6, 7, 8, 9]. Particularly in young adults, a corrective osteotomy is indicated to avoid salvage procedures such as arthroplasty or fusion [10, 11]. In a corrective osteotomy, the pathological bone is cut and the resulting bone parts are correctly realigned (i.e., anatomical reduction), followed by stable fixation with an osteosynthesis implant to secure their position during bone healing. Treatment goal is the anatomy reconstruction—restoration of length, angular and rotational alignment, and displacement. However, osteotomy is technically challenging to manage operatively and hence it demands careful preoperative planning. It is necessary to assess the deformity as accurate as possible to achieve adequate restoration. Conventionally, the deformity is quantified based on comparison, either to the healthy contralateral side [6, 12] if available or to anatomical standard values otherwise .
Three types of deformities are frequently encountered in an isolated or combined form and different types of osteotomies are performed to correct them [13, 14]: Angular deformity causes non-anatomical angulation of the distal and proximal bone parts. In general, angulation can occur in any oblique plane between the sagittal and the anterior-posterior planes. Isolated angular deformities are surgically corrected by performing a wedge osteotomy. In a closing wedge osteotomy, the bone is separated with two cuts forming a wedge which, when removed and closed, will correct the deformity. In contrast, in an opening wedge osteotomy one cut is made and the bone parts are aligned, resulting in a wedge-shaped opening. If the resulting gap is too large, which may result in implant failure due to instability, it is filled with structural bone graft to support healing before the parts are fixated . A rotational deformity is characterized by an excessive rotation or twist of the bone around its longitudinal axis. That is, the distal segment shows a non-anatomic rotation around its own axis while the proximal part is considered as fixed. Rotational deformities can be corrected by performing a derotation of the fragment, often within a single osteotomy plane. If a translational deformity is present, the pathological bone is longer or shorter than it anatomically should have been. Accordingly, a lengthening or shortening osteotomy must be performed.
In practice, most deformities are a combination of the three types of isolated deformities, which makes preoperative planning and the surgery very challenging. A special type of long bone deformities are intraarticular malunions [16, 17], i.e., non-anatomical healing of the bone after joint fracture, resulting in gaps or steps on the articular surface. Intra-articular malunions can cause severe damage to the cartilage, making surgical treatment often necessary although the intervention is risky.
Conventionally, deformity assessment relies on plain radiographs or single computed tomography (CT) slices. Angular deformities and length differences are manually measured on plain X-rays in antero-posterior and lateral projections [6, 7]. In CT or magnetic resonance imaging (MRI), rotational deformities are assessed using proximal and distal cross-sectional images to compare torsion between the sides. The problem of the conventional technique is the assumption that a multi-planar deformity can be corrected by subsequently correcting the deformities measured in the anterior-posterior, sagittal, and axial planes separately, which is indeed an invalid assumption . Accordingly, Nagy et al.  proposed to calculate the true angle of deformity in 3D space from planar measurements. Although feasible for angular deformities, their approach does not consider the rotational and translational components of a deformity.
Due to the above limitations of traditional approaches, computer assisted planning in 3D has become popular in orthopedic surgery as it permits to quantify a deformity by all 6 degrees of freedom (DoF)—3 translations and 3 rotations. Additionally, in contrast to emergency orthopedic treatments, corrective osteotomies are elective procedures scheduled in advance, therefore providing ample time for computer assisted planning. Since the first 3D osteotomy planning systems for the upper [19, 20] and lower extremities  had been described in the 90s, numerous approaches have been proposed for the 3D preoperative planning of long-bone deformities, i.e., osteotomies of the forearm bones, humerus, femur, and tibia. In these approaches, the basis for preoperative planning is a 3D triangular surface model of the patient anatomy generated from X-ray, CT, or MRI data of the patient. Based on the extracted 3D model, a preoperative plan can then be created by quantifying the deformity in 3D, followed by simulating the realignment to the normal anatomy. Dependent on the pathology and anatomy either the healthy contralateral limb, a similar-sized bone template, or anatomical standard values (e.g., axes, distances, and angles) can be used to determine normal anatomy. From a technical point of view, the alignment process to the normal anatomy using registration algorithms received most attention because the optimization target is easy to quantify. Apart from the fact that automatic alignment often achieves poor results, other aspects, such as the position of the osteotomy, are more difficult to define in a standardized way as they do mainly rely on clinical parameters and the experience of the planner. However, all pre- and intra-operative aspects have to be jointly considered for developing a clinically feasible plan. Moreover, each deformity is unique and for this reason the surgical strategy differs from patient to patient, which complicates the development of standardized approaches.
Performing the surgery according to a complex preoperative plan can also be challenging. Missing anatomical reference points due to limited access to and view of the bone makes it almost impossible to perform a 6-DoF reduction without any supporting equipment. Therefore, navigation systems were proposed to execute the preoperative plan intra-operatively. Navigation systems have been used among different disciplines of orthopedic surgeries , particularly for bone realignment of the upper  and lower extremities [21, 24]. Although they offer superior accuracy , navigation systems have been losing popularity since they are costly and laborious to use. Originating from dental surgery  and thanks to recent advances in additive manufacturing, the production of patient-specific instruments has instead become a cost-effective and promising alternative to navigation systems . Although the use of patient-specific guides for corrective osteotomies of long bones was described [11, 18, 28, 29, 30, 31, 32, 33, 34], the main application of patient-specific guides in orthopedic surgery is currently arthroplasty . The key idea behind patient-specific guides is to design targeting devices specific to the anatomy and pathology of the patient for guiding the surgeon intra-operatively. Such patient-specific instruments are extremely versatile and, hence, particularly suited for the treatment of bone deformities because the surgical treatment varies between patients.
Within the last few years, we have successfully treated more than 150 patients by corrective osteotomy using a computer assisted approach. In this chapter, we report on the techniques developed and the experience gained in this field. First, in Sect. 2, the 3D preoperative planning process is described step-by-step, providing a guideline for performing this task in a standardized way. In Sect. 3, current patient-specific instruments required for performing different types of osteotomies are summarized. In Sect. 4, the treatment of intra-articular osteotomies using 3D planning and patient-specific instruments is demonstrated by means of a complex case. Lastly, our results on the accuracy and effort are given in Sect. 5 including a discussion of advantages and limitations of the presented techniques.
2 Pre-operative Planning and Surgical Technique
The most fundamental step of the computer-based planning is the quantification of the deformity in 3D. Dependent on the anatomy and pathology, we either use a template-based approach or an axis-based approach to determine the deformity and, subsequently, the required correction.
2.1 Template-Based Approach
Template-based approaches can be performed if the opposite limb is uninjured. In contrast to the lower limb, bilateral deformities are rarely observed for the upper extremities, making this approach particularly suited for the planning of deformities of the forearm (i.e., radius and ulna) and shoulder bones. As in the conventional method, computer assisted planning relies on the contralateral bone, which serves as a reconstruction template. The input data for the planning tool are 3D models generated from CT scans of the posttraumatic bone as well as the contralateral (healthy) bone. The segmentation is performed in a semi-automatic fashion using thresholding and region-growing algorithms of commercial segmentation software (Mimics, Materialise, Loewen, Belgium). Triangular surface meshes are generated from segmented CT scans using the Marching Cube algorithm . Thereafter, the models are imported in our in-house developed planning-software CASPA (Balgrist CARD AG, Zurich, Switzerland).
A computer-planned corrective osteotomy does not involve only determining the optimal reduction, but also additional clinical factors crucial for a successful outcome shall be considered and optimized. After preliminary alignment, the surgeon is able to specify these parameters and constraints; e.g., possible access to the bone, osteotomy site, and type of the implant. Based on such specifications, the optimal surgical parameter values are determined in the planning tool: Besides the correction amount, the optimal selection of the osteotomy type, the position/orientation of the cutting plane(s), and the implant/screws position are important for a successful outcome and subsequent bone healing process.
The type of osteotomy and the orientation of the osteotomy plane directly influence how the bone surfaces will be in contact after reduction. Achieving maximum contact between the cortical bone layers of the fragments is strongly desired to promote healing. Again, the previously calculated axis (r, C) can provide a starting point to improve such contact.
As demonstrated in Fig. 3, a closing or opening wedge osteotomy is indicated if angular deformity is predominant; i.e., if the direction r is considerable different from the direction of the bone long axis. Adding or removing a bone wedge is also indicated if shortening or lengthening of the bone is required, i.e., a translation along the bone length axis between the aligned bone fragments was assessed. Given the osteotomy plane P and transformation T, the wedge is defined by P and \(T \cdot P\), where the line of intersection between the planes represents the hinge of the wedge. As demonstrated in Fig. 3b, so-called incomplete closing wedge osteotomies can be preoperatively planned if P is oriented such that the line of intersection coincides with (r, C) Incomplete osteotomies are often preferred for bones of the lower limb, promoting bone consolidation and early loading. In this procedure, the bone is not entirely cut but a small cortical bone bridge remains, almost constraining the reduction to a rotation around the bone bridge (i.e., the hinge of the wedge). Vice-versa, the preoperatively planned reduction can be only achieved if the hinge corresponds to (r, C). More examples of this osteotomy type will be given later in Sect. 2.2. Dependent on the position of the hinge, a situation may arise where the wedge type is not clearly defined; i.e., part of the bones overlap while also a gap is created. For such situations, CASPA permits the visualization of the wedge in real time by simultaneously rendering P and \(T \cdot P\) on display. By doing so, the osteotomy plane can be interactively translated until the desired wedge osteotomy can be achieved.
If a rotational deformity is predominant, a so-called single cut osteotomy may be preferable to a wedge osteotomy. In this type of osteotomy, a single cut is performed which is pre-calculated such that sliding along and rotation in the cut plane permits reduction as planned . An example of this osteotomy type is given in Fig. 3c. The position and orientation of a single cut osteotomy plane can be derived directly from the calculated reduction, i.e., the plane normal is defined by r. Although the single cut plane can be calculated exactly for any T, the osteotomy can be only applied if the translational component of T along the plane normal is negligible (i.e., below 1 mm) and the tangential translation is small, because otherwise either the bone contact surface would be too small or, worse, a gap between the bone parts would be created.
The inclusion of the osteosynthesis plate model, if available, into the preoperative plan, as demonstrated in Fig. 8a, can help to ensure a proper fitting of the plate. A poor plate fitting may result in less controlled procedure and decreased stability, which may lead to inaccurate reduction, delayed or stalled bone healing, or even implant failure. Inappropriately placed screws are another cause of delayed consolidation, and they may also harm the surrounding soft tissue. The best-fitting implant type can indeed be easily determined in the planning tool and also the optimal position of such implant can already be defined preoperatively. Optimally, the implant also contains the position and direction of the screws. The screw models are used by the surgeon to verify that all screws will be placed sufficiently inside the bone but without penetrating the osteotomy plane or anatomical regions at risk.
2.2 Axis Realignment
Realignment of the lower limb is primarily indicated in patients having a varus- (out-knee) or valgus-deformity (in-knee), resulting in a pathologically deviated weight-bearing line of the leg (i.e., the mechanical axis) . In many cases, the deformity is caused by a congenital defect affecting both limbs and, therefore, the contralateral bone cannot be used as a reconstruction template. Instead, the required correction is determined by realigning the mechanical axis to its anatomically normal position. According to Paley et al. , the mechanical leg axis passes not exactly through the knee joint center, but is located 8 ± 7 mm medial to the knee joint center. This deviation is called mechanical axis deviation (MAD) and is measured as the perpendicular distance from the mechanical leg axis to the knee joint center. A malalignment can be considered as pathological if the MAD lies outside of this normal range.
We have developed a method for quantifying the required 3D osteotomy parameters (i.e., position, rotation axis, and angle). Our approach requires having triangular surface models of the hip, knee, and ankle joints. We apply a CT protocol that scans the regions of interests while skipping the irrelevant mid-shaft regions to reduce radiation exposure. Based on the CT scan, a limb model consisting of the proximal femur, distal femur, patella, proximal tibia, distal tibia, distal fibula and talus, is reconstructed. Thereafter the hip, knee, and ankle joint centers of the limb model have to be determined in 3D before the mechanical axes can be calculated.
3 Surgical Technique and Guides
Creation of a computer assisted surgical plan and the implementation of the plan using a specific surgical technique go hand in hand and cannot be considered separately. On one hand, the surgical technique and strategy must be known before developing a preoperative plan. On the other hand, it must be possible to realize a preoperative plan without risk and with state-of-the-art surgical techniques. For multi-planar deformities, the reduction task is often too complex to be performed conventionally without additional tools supporting the surgeon. We use patient-specific instruments in the surgery, enabling the surgeon to reduce the bone exactly as planned on the computer. These so-called surgical guides have proved successful over years in different orthopedic interventions. The basic principle is that the body of a guide is shaped such that it can be uniquely placed on its planned position on the bone by using characteristics of the irregular shaped bone surface and it helps intra-operatively to define the osteotomy plane and correction.
Each malunion and, consequently, each surgery is different which makes not only preoperative planning but also patient-specific instrumentation challenging. The goal is to develop guides that are sufficiently flexible to be applied to various types of osteotomies, but nevertheless enabling generation in a standardized and time-efficient way. In our experience, patient-specific guides have to be considered as an integral part of the surgical plan; i.e., they should be designed in the planning tool to define the osteotomy position, the direction of the cut, the position of the implant, and the relative transformation of the fragment after reduction. The CASPA tool enables the generation of the guides in a semi-automatic fashion. In a first step, the guide body is created based on a 2D outline drawn around the osteotomy location on the bone surface. It is crucial that this step is performed by a surgeon, because the guide surface must not cover soft-tissue structures (e.g., ligaments) that cannot be removed from the bone for guide placement. To generate a 3D guide body, the outline is extruded normal to the bone surface by a user defined height, followed by boolean subtraction of the bone surface . To achieve a unique fit, the shape of the guide body is designed to contain irregular convex and concave parts covering the bone from different directions.
Using additive manufacturing, patient-specific instruments can be produced based on 3D models designed in the planning application. In our case, the guides are manufactured by Medacta SA (Castel San Pietro, Switzerland) using a selective laser sintering device (EOS GmbH, Krailling, Germany). The guides are made of medical-grade polyamide 12. They are cleaned with a surgical washer and sterilized using conventional steam pressure sterilization at our institution.
4 Intraarticular Osteotomies
Corrective osteotomies of intra-articular malunions are among the most challenging orthopedic procedures with respect to both preoperative planning and surgery. The limited access to the joint surface makes an inside-out-osteotomy (i.e., cutting from the joint surface towards the shaft) difficult or, in certain settings, even impossible. An extra-articular, outside-in approach to the intra-articular malunion provides an alternative in the surgical treatment. However, such a technique requires extensive preoperative planning and, moreover, the surgeon must be guided intraoperatively because he has no direct view into the joint. We have developed a preoperative planning methodology combined with a closely-linked surgical technique , enabling the surgeon to perform an outside-in approach in a controlled way. We will describe the approach based on one of the most complex cases treated at our institution so far.
5 Results and Discussion
In this chapter, we have described a technique combining 3D preoperative planning of long bone deformity correction with additive-manufactured patient-specific instruments. So far, we have applied the approach to osteotomies of the radius, ulna, humerus, femur, tibia, and fibula. The method has enabled us to perform the osteotomies more accurately and in a more controlled fashion.
Exact restoration of the normal anatomy is crucial for a satisfactory clinical outcome of orthopedic surgeries . We also consider the accuracy of the procedure, i.e., how precisely the reduction is performed compared to the preoperative plan, as one of the major outcome measures. Besides demonstrating efficacy of the presented technique, accuracy evaluation provides the surgeon a quantitative control of success, enabling the minimization of technical errors and improvement of surgical skills. The accuracy of the surgical procedure can be assessed by comparing the desired planning result with the reduction performed in the surgery based on postoperative images. As in the preoperative planning, CT-based 3D evaluation is considered as the gold standard for assessing the residual deformity [11, 50]. In our case, postoperative CT images were available in several cases, acquired to assess bone consolidation during routine clinical follow-ups. Using the postoperative bone model extracted from CT, the same registration method as in the preoperative planning can be applied to quantify the difference between planned and performed reduction in 3D. We have performed CT-based accuracy evaluation of 37 surgeries, comprising different types of osteotomies and anatomy. The residual rotation error is expressed by the 3D angle of the rotation in axis-angle representation. The residual translation error is given by the length of the 3D displacement vector.
Computerized preoperative planning offers high accuracy and satisfactory results for surgical outcome; nevertheless, it still presents some challenges to be considered carefully. Preoperatively defining the reduction target (i.e., the goal model) correctly is a major contributor to a successful planning. So far, the contralateral bone has been proved to be the best reconstruction template available [12, 18] although considerable bilateral asymmetry may exist [39, 40, 41]. Even in case of perfect symmetry, soft tissues may have a considerable influence on the joint function. Consequently, a successful clinical outcome cannot always be ensured by merely relying on the contralateral bone anatomy, even if the reduction is performed precisely as planned. The integration of a pre- and postoperative motion simulation into the preoperative planning application may be the next step to better predict the functional outcome  for soft tissue injuries. Apart from this, the resolution of the image data used for planning and the subsequent segmentation process are additional technical factors, which may also influence the planning accuracy. Nevertheless, bone models extracted from CT scans with an axial resolution of 1 mm and using interactive segmentation methods, such as thresholding and region growing, have been shown to be sufficiently accurate for preoperative planning . Intraoperatively, the correct placement of the guide(s) on the bone also has a major impact on the accuracy of the entire procedure, because the reduction is computed relative to the osteotomy site. Therefore, all means should be taken to ensure a precise identification of the intended guide position intra-operatively. One first step verification is to check the fit of the guide on real-size replica of the bone anatomy, manufactured with laser sintering devices. For a precise fit in the surgery, the bone should be debrided from periosteum as much as possible. Nevertheless, for finding a stable fit on regularly shaped surfaces such as the cylindrical mid-shaft region, manual measurements with respect to anatomical reference points may still be necessary, which is a limitation of the presented method.
We have had different experiences regarding the two different types of reduction guides described in Sect. 3. Using the technique based on pre-drilling the screw holes of the implant, we have observed some difficulties in achieving the correct reduction for osteotomies where extensive soft tissue tension was present (e.g., opening wedge distal radius or tibia osteotomies). Particularly in these cases, separate reduction guides appeared to be more suited and more accurate. An additional benefit of separate reduction guides is that dedicated parts of the guide body, such as wedges, can be used in addition to the K-wires to further improve the control of the reduction. However, guide generation is more time-consuming for multi-part reduction guides and the exposure of the bone must be larger. Due to the limited access to the bone, the separate reduction guide and additionally required K-wires may complicate surgical procedures such as sawing or implant fixation. In the future, patient-specific osteosynthesis plates with additional functionality, such as supporting the surgeon in the reduction, may be a promising alternative to combine the advantages of the present guiding techniques. Moreover, the possibility of using patient-specific osteosynthesis implants, integrated into the preoperative plan and fitted to the patient anatomy, would also improve the evident problem of the poor fitting of anatomical standard-plates. Recently, the application of a first patient-specific implant prototype has been demonstrated in-vitro . However, its high manufacturing costs over 1000 USD limit its wide-spread clinical application. Selective laser melting, which has proven to be highly productive in other medical fields, may allow for a cost-effective fabrication in the future.
Average time necessary for generating a computer-based preoperative plan for three different categories of complexity
Simple: opening-wedge or single-cut shaft osteotomies
Medium: closing wedge osteotomies, two-bone osteotomies (radius-ulna, tibia-fibula)
Complex: intra-articular osteotomies, double osteotomies
Simple and standardized: one pre- and one postoperative guide
Medium: three or more guides
Complex: more than three, highly individualized guides
Average planning time (h)
2.1 ± 0.8 h (n = 16)
4.0 ± 0.9 h (n = 20)
9.1 ± 3.7 h (n =28)
Compared to the conventional approach, an additional cost of €250 ($340 USD) per case arises due to guide manufacturing. Moreover, the radiation exposure for the patient may be higher due to the increased use of preoperative CT imaging. Nevertheless, the technique may reduce total fluoroscopy time in the surgery.
In conclusion, 3D planning has become an integral part of the preoperative assessment for long bone deformities at our institution. While simpler corrective osteotomies can be efficiently planned in a standardized way using dedicated planning tools, more complex cases still require a laborious manual effort, resulting in considerable personnel costs. Considering these cases, further research must be performed to reduce the preoperative planning time. Additive manufacturing has revolutionized computer-assisted surgery. Patient-specific surgical guides provide an efficient and accurate way of implementing a computer-based surgical plan in the surgery. As the preoperative plan and the surgical technique are closely linked, training of the surgeons in using surgical guides is essential. After gaining practical experience, the technique may reduce operation times and enables osteotomy corrections that were not possible before, e.g., complex intra-articular osteotomies . As a next future step, the extension of the method to other anatomy, such as bones of the wrist or foot, will be studied.
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