Surgical Technique of the 3-Dimensional-printed Personalized Hip Implant for the Treatment of Canine Hip Dysplasia

This study was considered a non-experimental clinical veterinary practice as mentioned in Article 1 – 5(b) of Directive 2010/63/EU and was approved by the Veterinary Clinical Studies Committees (VCSC), Utrecht University, Utrecht, The Netherlands. This study involved the treatment of client-owned dogs, with all dogs continuing under the care of their respective owners. All dog owners were provided with an information letter detailing the study protocol, all potential complications (e.g., infection, implant failure, neurological deficits, and others), and alternative treatments like pelvic osteotomy. Furthermore, in this form, privacy aspects and inherent data management were explained. All clients signed an informed consent form. The entire protocol of this study is divided into the following major steps: patient selection, 3DHIP implant design and production, preoperative management and anesthesia, surgical procedure, and post-operative management.

1. Patient selection

1. Identify client-owned dogs > 6 months old with clinical signs related to HD that have a positive Ortolani subluxation sign (Figure 1) and radiographic evidence of HD with FCI grade B to D (Figure 2).
   NOTE: The Ortolani subluxation test is a diagnostic maneuver used in veterinary medicine to assess hip joint stability in dogs. During the Ortolani test in dogs, the examiner positions the animal on its back and flexes the hips to 90° while stabilizing the pelvis. Each hind limb is gently abducted, aiming to displace the femoral head from the acetabulum to assess hip joint stability. A positive Ortolani sign, indicated by a characteristic "clunk" or movement, suggests hip dysplasia and the potential for hip joint instability.
2. Exclude the dogs with an open acetabular growth plate, luxoid hips, or prior hip surgery.
3. Perform a CT scan of the hips to exclude dogs with moderate to severe osteoarthritic changes of the hips and for implant design.
   NOTE: Dogs that have femoral neck and/or cranial and caudal acetabular rim osteophytes > 2 mm are excluded (Figure 3).

2. 3DHIP implant design and production

1. Design the 3DHIP implant (patent number EP3463198B1¹⁹) from preoperative CT DICOM images of the complete pelvis as described by Willemsen et al.¹⁶ and Kwananocha et al.¹⁸ at the laboratory (refer to the Table of Materials).
   1. Segment the pelvic bone and femora of each candidate dog from the preoperative CT and create a 3D model using specialized software (refer to the Table of Materials and Figure 4A).
   2. Create a local coordinate system for the pelvis, based on the inclinated posterior pelvic plane.
   3. Measure the native Norberg angle (NA) of each candidate dog's hips on the 3D model (Figure 4B).
   4. Use the specialized software (refer to the Table of Materials) to design the 3DHIP implant on the 3D model of the pelvis. Use the native NA to determine the amount of extension of the dorsal acetabular rim that is needed; the 3DHIP implant increases the NA by 25-35 degrees (Figure 4C and Figure 4D).
   5. Ensure that the 3DHIP implant consists of two subsections: the attachment part and the rim extension part. Design the attachment part to consist of a porous inner shell and incorporate four locking screw holes and an additional ventral ilium flange for ease and precision of positioning. Design the rim extension part to extend the dorsal acetabular rim with an internal 1.5 mm offset allowing unhindered joint capsule attachment (Figure 5A and Figure 5B).
2. Print the created 3DHIP implant (Figure 5C) from medical grade titanium alloy (Ti6AI4V ELI grade 23) using selective laser melting technology with a direct metal printing machine, operated by the implant manufacturer (refer to the Table of Materials).
3. Postprocess the printed 3DHIP implant, including stress release (vacuum annealing), manual mirror polishing, and ultrasonic cleaning, operated by the implant manufacturer (refer to Table of Materials).
4. Perform final cleaning at the hospital sterilization unit to eliminate any residual metal dust leftover from the manufacturing process.
   1. Manually wash the 3DHIP implant using povidone-iodine shampoo (refer to the Table of Materials) and sterile water.
   2. Clean the 3DHIP implant using the hygienic washing machine (refer to the Table of Materials) at a temperature of 94 °C for 90 min.
5. Enclose the 3DHIP implant in double transparent sterilization laminates (refer to the Table of Materials) and seal each laminate separately.
6. Sterilize the implant using steam autoclaving (refer to the Table of Materials) at a temperature of 134 °C for 80 min.

3. Preoperative management and anesthesia

1. Perform a general examination of the client-owned dog before anesthesia and categorize the anesthesia risk using the American Society of Anesthesiologists (ASA) patient scale (scale 1-5)²⁰.
   NOTE: The American Society of Anesthesiologists (ASA) patient scale is employed to categorize the anesthesia risk of dogs and assists veterinarians in determining appropriate anesthetic protocols for each dog. It classifies dogs into different classes based on their health status, ranging from Class I for healthy dogs to Class V for those in critical condition not expected to survive surgery. Dogs categorized under ASA 1 (normal healthy dog) and ASA 2 (dogs with mild systemic disease) are considered suitable candidates for this surgical treatment. These dogs typically have no or mild underlying systemic diseases, are in good overall health, and exhibit normal physiological functions. They are considered low-risk candidates for surgery and anesthesia. The animal needs to be fasted for at least 6 h before the scheduled induction time.
2. Anesthetize the dog following ASA categorization on the individual patient's needs.
   1. Administer intravenous dexmedetomidine at a dose of 2 µg/kg and methadone hydrochloride at a dose of 0.3 mg/kg for premedication in the dog. (refer to the Table of Materials).
      NOTE: The selection of premedications and the route of administration may differ depending on the anesthesiologist's preference and the dog's health condition and behavior.
   2. Administer intravenous propofol at a dosage of 2-4 mg/kg (refer to the Table of Materials) for anesthesia induction.
   3. Intubate the dog and sustain anesthesia with inhaled isoflurane (refer to the Table of Materials) and oxygen.
3. Continuously monitor and ensure the stability of vital signs, encompassing heart rate, respiratory rate, end-expiratory carbon dioxide levels, percutaneous arterial oxygen saturation, non-invasive arterial blood pressure, esophageal temperature, and electrocardiography.
4. Perform epidural analgesia using a sterile technique.
   1. Position the dog in sternal recumbency and gently flex the hind limbs forward to create more space between the last lumbar vertebra and the sacrum.
   2. Identify the injection site located just caudal to the 7th dorsal spinal process, where a "dimple" can be palpated.
   3. Conduct a sterile scrub and don sterile gloves using aseptic techniques.
   4. Insert the spinal needle tip into the epidural space and verify its correct placement using the "hanging drop" technique. Briefly, introduce a drop of saline into the spinal needle hub. When the spinal needle tip penetrates the ligamentum flavum and enters the epidural space, the saline within the hub will migrate from the hub into the needle²¹.
   5. Inject the drugs (morphine 0.1 mg/kg diluted with levobupivacaine 1 mL/5 kg) (refer to the Table of Materials) at a slow, constant rate to ensure even distribution of the drug when correct placement is confirmed.
5. Place an indwelling Foley urinary catheter (refer to the Table of Materials) in the bladder using a sterile technique and leave it in place for 12-24 h.
   1. Ensure a clean and sterile environment for the catheterization procedure; trim the hair on the prepuce in male dogs and the surrounding ventral vaginal vault in female dogs.
   2. Cleanse the area with povidone-iodine solution (refer to the Table of Materials) and flush the prepuce/vaginal vault with 2-12 mL of diluted povidone-iodine solution (the volume varies based on the size of the dog).
   3. Wash hands thoroughly and wear sterile gloves to minimize the risk of contamination.
   4. Apply sterile lubricating jelly to the distal end of the Foley catheter and employ aseptic technique during the insertion of the Foley catheter.
   5. Once the catheter is correctly positioned in the bladder, inflate the balloon with sterile water according to the volume specified on the package. This secures the catheter in place and prevents accidental dislodgement.
   6. Connect the drainage port on the Foley catheter to the receiving port on a urine collection bag.
6. Clip the entire limb circumferentially starting from the vertebral column to just distal to the hock. In case of single-stage bilateral 3DHIP implantation, clip the other limb in a similar fashion and connect the left and right sides on the lumbosacral dorsum.
7. Wrap the distal part of the limb with a non-sterile cohesive bandage. Select an appropriate width and length of non-sterile cohesive bandage, starting slightly below the surgical site and wrapping it spirally down the limb, covering the paw and nails.
8. Administer cefazolin for injection at a dose of 20 mg/kg (refer to the Table of Materials) intravenously 30 min before skin incision and repeat every 90 min until the end of surgery.
9. Position the dog on a standard operating table in lateral recumbency and put the affected limb into a hanging position. Secure the patient in this position using a vacuum bean bag positioner
10. Conduct the final sterile surgical scrub on the limb in preparation for the surgery. Scrub the skin 2x with 4% chlorhexidine gluconate, and finish with two applications of 70% (v/v) ethanol spray (refer to the Table of Materials).
11. Position four surgical waterproof drapes around the surgical site. Direct an assistant to release the distal limb from its suspended position, while the surgeon secures and covers the distal limb with a sterile waterproof sock. Add a layer of sterile cohesive wrap for extra protection.
12. Cover the exposed skin of the entire surgical area with an iodine-impregnated drape (refer to the Table of Materials) and then secure with towel clamps.

4. Surgical procedure

1. Identify the tip of the greater trochanter, the cranial border of the proximal femur, and the iliac wing through palpation to establish orientation.
2. Incise the skin sharp with a surgical knife from the cranial dorsal iliac spine starting 6-10 cm cranial to the greater trochanter. Then, slightly turn ventrally along the cranial border of the proximal femur. Stop the incision 2-5 cm distal to the greater trochanter. The incision length is approximately 8-15 cm (depending on the size of the dog; Figure 6A).
   NOTE: This surgical approach was modified from the craniodorsal approach of the hip joint previously reported by Johnson²².
3. Make an incision through the subcutaneous fat down to the fascia to establish an anatomic dissection following the anatomic planes (Figure 6B).
4. Sharply separate and incise the superficial leaf of the fascia latae muscle along the cranial border of the biceps femoris muscle. Retract the biceps femoris muscle caudally.
5. Identify the fatty triangle, which is bordered by the tensor fascia latea muscle, the gluteal muscle, and the biceps femoris muscle. Separate the fatty triangle with a blunt-tipped dissecting scissor and index finger, which will provide access to the deeper layers.
6. Incise the intermuscular septum between the superficial gluteal muscle, the middle gluteal muscle, and the tensor fascia latae muscle with a surgical knife (Figure 7A and Figure 7E).
7. Use a hand-held retractor to separate and retract the superficial and middle gluteal muscles dorsally, which will expose the insertion of the deep gluteal muscle.
8. Undermine the deep gluteal muscle close to the greater trochanter using blunt-tipped dissecting scissors.
9. Preplace a stay suture on the deep gluteal tendon approximately 1-1.5 cm proximal from its insertion on the greater trochanter.
10. Perform a complete deep gluteal tenotomy close to the bone (approximately 0.5-1 cm away from its insertion) using a surgical knife (Figure 7B and Figure 7F).
11. Do a blunt dissection using blunt-tipped dissection scissors to free the deep gluteal muscle from the underlying joint capsule, following which it can be subperiosteally elevated from the ilium using a periosteal elevator and index finger.
12. Use bipolar electrocautery for hemostasis of small vessels between the deep gluteal muscle and the joint capsule. Then, retract the deep gluteal muscle dorsally by replacing the Amry Navy retractors.
13. Partially free the iliacus muscle from the caudoventral border of the iliac shaft using a periosteal elevator and identify the insertion of the rectus femoris muscle (Figure 7C and Figure 7G).
14. Use a periosteal elevator to remove all remaining soft tissue from the exposed iliac shaft to prepare for accurate positioning of the 3DHIP implant and scratch the periosteum to stimulate bone ingrowth for secondary fixation of the implant.
15. Identify the articularis coxae muscle caudal to the rectus femoris muscle overlying the joint capsule.
    NOTE: The articularis coxae muscle can be freed from its insertion if it interferes with implant positioning.
16. Fit the 3DHIP implant into its designated position with the implant's flange of the attachment part hooking under the ventral border of the exposed caudoventral iliac shaft just cranial to the bony prominence that marks the insertion of the rectus femoris muscle (Figure 7D and Figure 7H).
17. Check that the rim extension part of the implant overlays the craniodorsal part of the hip joint capsule without interfering with the attachment of the joint capsule on the acetabular rim and that no deep gluteal muscle is captured under the extension part.
    NOTE: Adequate exposure for implant positioning and screw insertion can be achieved by continuous abduction, external rotation, and flexion of the hip joint by an assistant to release the tension on the gluteal muscle complex facilitating the surgical exposure.
18. Check the implant for ideal positioning by visualizing and probing with the suction canula for perfect bone stock in all four exposed screw holes and by probing for the absence of space between the ilial flange and the caudoventral iliac shaft.
19. Temporarily fix the implant in the desired position with one titanium self-tapping locking screw (2.4 mm, 2.7 mm, or 3.5 mm) (refer to the Table of Materials), which is not fully tightened to allow minimal rotational adjustments to the implant when placing the second screw.
    NOTE: The sequence of placement of the four screws can be adapted according to their convenient accessibility (Figure 5C).
20. Conduct intraoperative fluoroscopy (refer to the Table of Materials) in lateral (Figure 8A) and latero-oblique (Figure 8B) views to comprehensively assess the implant's position and alignment. Compare the obtained fluoroscopic images with preoperative planning to ensure that the implant is positioned according to the surgical plan.
21. Check that the curvature of the rim extension part of the implant is exactly congruent with the curvature of the femoral head and visible caudal and cranial acetabular rim that are not covered by the implant.
    NOTE: If necessary, adjustments can be performed. The first screw is removed, the implant replaced and temporarily fixed with one screw in new bone stock, and fluoroscopy is repeated.
22. Insert three titanium self-tapping locking screws (2.4 mm, 2.7 mm, or 3.5 mm) in the remaining screw holes to fix the implant to the ilial shaft. After the second screw is placed, completely tighten the first screw.
23. Perform a final check that all screws are hand-tightened to the locking mechanism.
24. Perform flexion, extension, and abduction of the hip joint as well as the Ortolani subluxation test to exclude femoral head/neck impingement and ensure that hip laxity is reversed.
25. Re-attach the cut ends of the insertional tendon of the deep gluteal muscle using a locking loop suture pattern and 1-2 mattress sutures with synthetic absorbable monofilament suture material (refer to the Table of Materials) that are intended for approximation of tissue for extended periods. Extend and internally rotate the hip joint to decrease tension on the insertional tendon of the deep gluteal muscle while suturing.
26. Repair the gluteal fascia and tensor fascia latae with synthetic monofilament absorbable suture in a simple interrupted pattern.
27. Close the subcutaneous tissue with synthetic monofilament absorbable suture in a simple interrupted pattern and close the skin with synthetic monofilament non-absorbable suture in a simple interrupted pattern (refer to the Table of Materials).
    NOTE: If a single-stage bilateral procedure is planned, turn the dog to the other side with the untreated limb in a hanging position. Following aseptic preparation, surgical procedure steps 3.10-4.27 are performed in a similar fashion.
28. Perform postoperative imaging consisting of either a CT scan of the hips or hip orthogonal radiographs in latero-oblique and ventrodorsal views for final evaluation of implant positioning and screw placement (Figure 9).

5. Postoperative management

1. Provide overnight inpatient care and pain management (e.g., with continuous rate infusions [CRI] of ketamine HCl 2-10 mcg∙kg^-1∙min^-1 or sufentanil citrate 0.1 mcg∙kg^-1∙h^-1 with methadone (IV, 0.2 mg/kg every 6 h, carprofen (IV, 4 mg/kg once daily), and gabapentin (PO, 10 mg/kg every 8 h) (refer to the Table of Materials).
2. Allow direct postoperative short leash walking as needed to urinate and defecate on the non-slippery floor on the following day (see Supplementary Video S1).
   NOTE: In case of hindlimb instability or problems rising, use a support sling or towel under the belly to support the hindlimbs.
3. Allow home discharge the day after surgery after removal of the Foley catheter and when voluntary urination is observed.
4. Prescribe medications for pain management at home (e.g., oral medications such as carprofen 2 mg/kg BID and gabapentin 10 mg/kg every 8 h for 14 days) (refer to the Table of Materials).
   NOTE: In hyperactive dogs, trazodone hydrochloride 2-5 mg/kg BID (refer to the Table of Materials), oral can be administered. This can be continued for 1-2 weeks postoperatively to ensure safe rehabilitation without high impact activities.
5. Prohibit high-impact activities (e.g., jumping, running, stair climbing, running with other pets, or 'rough housing') for 6 weeks postoperatively. At home, walk dogs slowly and encourage them to use the operated hindlimb(s) according to a weekly exercise scheme. Within 6 weeks postoperatively, allow the dog patient to walk with leash restraint 4-6 times daily, initially for 5-10 min each in the first 2 weeks, and then extend the duration by 5 min every 2 weeks.
6. Advise professional physiotherapy and/or hydrotherapy from the second week after surgery when skin wound healing is complete.