Pericapsular hip desensitization in dogs: a cadaveric study and case series

ObjectiveTo develop and assess the efficacy of an ultrasound (US)-guided pericapsular hip desensitization (PHD) technique in dogs.Study designProspective, randomized, anatomical study and a case series.AnimalsA total of 30 healthy dogs, eight canine cadavers and seven dogs with hip osteoarthritis.MethodsAfter studying the US anatomy of the medial aspect of the coxofemoral joint and determining an acoustic window to perform an US-guided PHD in healthy dogs, the US-guided PHD was performed bilaterally in canine cadavers. A low [(LV) 0.1 mL kg^–1] and high [(HV) 0.2 mL kg^–1] volume of dye was injected per hip on each cadaver. The staining of the pericapsular nerves was assessed by anatomical dissection, and comparison between LV and HV was assessed using Fisher’s exact test. Then, the US-guided PHD was performed using a triamcinolone–bupivacaine solution in dogs with hip osteoarthritis. Dynamic pain response was assessed before and after injection. The canine brief pain inventory (CBPI) questionnaire was used to assess treatment efficacy and duration.ResultsThe US-guided PHD could be performed by inserting the needle between the iliopsoas muscle and the periosteum of the ilium. The articular branches of the femoral and obturator nerves were stained in all cadavers using both volumes. The main femoral nerve was never stained, but the main obturator nerve was stained in 37.5% and 100% of injections using LV and HV, respectively (p = 0.026). Treated animals showed decreased dynamic pain response after the injection. Compared with baseline, CBPI scores were reduced by ≥ 50% for ≥ 12 weeks in all but one dog.Conclusions and clinical significanceThe US-guided PHD with both 0.1 and 0.2 mL kg^–1 volumes stained the articular branches of the femoral and obturator nerves in canine cadavers and was associated with clinical improvement in dogs with hip osteoarthritis.     

TECHNIQUE FOR ULTRASOUND‐GUIDED INTRAARTICULAR CERVICAL ARTICULAR PROCESS INJECTION IN THE DOG

Ultrasound‐guided intraarticular injection of cervical articular process joints is a well‐established procedure in both humans and horses for neck pain resulting from osteoarthritis, but the technique has not been described in dogs. Aims of this study were to describe the ultrasonographic anatomy and landmarks for cervical articular process joint injections in the dog, develop a technique for articular process joint injections using these landmarks, and determine the accuracy of injections and factors that may influence it. Eleven canine cadavers were used and bilateral joint spaces from C2–3 to C7‐T1 were injected under ultrasound guidance with a blue radiopaque solution. A computed tomographic scan was acquired following each injection, and an injection score was assigned and compared with other patient‐specific factors. Of the 132 injections performed, 110 (83.3%) were intraarticular, 20 (15.1%) were periarticular within 5 mm, and 2 (1.5%) were periarticular beyond 5 mm from the joint. There was no significant difference in mean scores between dogs. Only C2–3 had a significantly lower mean score than any other joint. There was no significant correlation between injection score and any other factors measured. The transverse processes of the cervical vertebrae served as excellent ultrasonographic landmarks for identifying the cervical articular process joints in dogs regardless of the size of the dog or location along the vertebrae. Accuracy of ultrasound‐guided intraarticular process joint injection was 83% in dogs and similar to published techniques in horses. Further studies are needed to examine the safety and efficacy of this procedure in live animals.     

FEASIBILITY OF ULTRASOUND‐GUIDED EPIDURAL ACCESS AT THE LUMBO‐SACRAL SPACE IN DOGS

Epidural injections are commonly performed blindly in veterinary medicine. The aims of this study were to describe the lumbosacral ultrasonographic anatomy and to assess the feasibility of an ultrasound‐guided epidural injection technique in dogs. A cross sectional anatomic atlas of the lumbosacral region and ex vivo ultrasound images were obtained in two cadavers to describe the ultrasound anatomy and to identify the landmarks. Sixteen normal weight canine cadavers were used to establish two variations of the technique for direct ultrasound‐guided injection, using spinal needles or epidural catheters. The technique was finally performed in two normal weight cadavers, in two overweight cadavers and in five live dogs with radiographic abnormalities resulting of the lumbosacral spine. Contrast medium was injected and CT was used to assess the success of the injection. The anatomic landmarks to carry out the procedure were the seventh lumbar vertebra, the iliac wings, and the first sacral vertebra. The target for directing the needle was the trapezoid‐shaped echogenic zone between the contiguous articular facets of the lumbosacral vertebral canal visualized in a parasagittal plane. The spinal needle or epidural catheter was inserted in a 45° craniodorsal–caudoventral direction through the subcutaneous tissue and the interarcuate ligament until reaching the epidural space. CT examination confirmed the presence of contrast medium in the epidural space in 25/25 dogs, although a variable contamination of the subarachnoid space was also noted. Findings indicated that this ultrasound‐guided epidural injection technique is feasible for normal weight and overweight dogs, with and without radiographic abnormalities of the spine.     

The Innervation of Canine Hip Joint Capsule: An Anatomic Study

To clarify the contributions of the nerves supplying the canine hip joint capsule for clinical application, cadaver study of six healthy mongrel dogs was performed. The pelvises and hindlimbs of cadavers were dissected and fixed in formaldehyde. Innervation of the joint capsule was investigated with the aid of an operative microscope. As a result, the canine hip joint capsule receives multiple innervations from articular branches of four nerves. They are articular nerve fibres of femoral, obturator, cranial gluteal and sciatic nerves from the cranioventral, caudoventral, craniolateral and dorsolateral directions of the joint, respectively. No branch originating from the caudal gluteal nerve was observed innervating the hip joint capsule. Our data provides useful information for research on the canine hip joint, including pain analysis with hip disorders and surgical nerve blockade to relieve pain.     

Ultrasonographic examination of the hip joint region and bony pelvis in cattle

Transcutaneous and transrectal ultrasonographic examination of the hip joint region and the pelvis was carried out in 7 cadavers, transcutaneous ultrasonography in 17 healthy young and adult cattle, and transrectal sonography was performed in 12 healthy cows in order to study the normal ultrasonographic appearance of these regions. 7.5 MHz linear-, 5.0 MHz and 3.5 MHz convex transducers and a 7.5 MHz rectal probe were used. The bone surfaces of the greater trochanter, the femoral neck and head, the acetabulum and the other pelvic bones were visualised as hyperechoic contours. The coxofemoral joint space was identified in all cadavers and live cattle. The joint pouch could not be visualised, neither in cadavers nor in live cattle. After experimental filling of the coxofemoral joint pouch by injection of 35-45 ml eosin-solution it appeared as a large anechoic zone between the articular surface and the echogenic joint capsule. The inner contours of the pelvic girdle, both iliosacral joint spaces, the abdominal aorta and the external iliac arteries were depicted clearly by transrectal ultrasonography. The practical application of diagnostic ultrasound in these regions is demonstrated in clinical patients suffering from a septic coxarthritis and a sequestration of a part of the tuber coxae following an open fracture. These results serve as reference data for ultrasonographic investigation of disorders of the hip joint and the pelvic region in cattle.     

双峰驼髋关节解剖特征

运用大体解剖学方法,结合活体Ｘ射线透视及光学摄影术,观察了双峰驼的髋关节。与马、牛、羊、猪、犬及人的对等器官比,双峰驼的髋关节囊纤维层增厚,形成髂股、坐股及耻股３条韧带,分别类似于人的同名韧带;关节囊纤维层有少量弓形及半环形纤维,但未形成轮匝带;股骨头韧带始于髋臼切迹及髋臼窝,与人（始于髋臼横韧带及髋臼切迹两侧）、马（始于近髋臼切迹的耻骨下沟）均不同;髋关节无副韧带（与牛同）。     

Effect of surgical technique and use of a rigid centralizing device on stem positioning and geometric reconstruction in the sagittal plane during total hip replacement in canine cadavers

OBJECTIVE: To determine the effect of surgical technique and use of a rigid centralizing device on stem positioning and geometric reconstruction in the sagittal plane during total hip replacement in dogs. SAMPLE POPULATION: Bilateral femurs from 8 adult mixed-breed canine cadavers. PROCEDURE: Femurs were prepared for femoral stem implantation, using 4 variations in technique. Proximal femoral reconstruction and femoral stem positioning were evaluated on radiographs. RESULTS: Implants evaluated in this study accurately reconstructed displacement of the femoral head of the intact canine femur in the sagittal plane. Centralization of the distal aspect of the stem was optimized by use of an undersized femoral stem. Ostectomy at the level of the lesser trochanter resulted in the smallest diaphysis-to-implant angle. Anteversion and retroversion of implants significantly decreased the distance between the distal tip of the implant and the adjacent cortex, compared with normoversion. The centralizing device significantly increased the minimum distance between the distal tip of the implant and adjacent cortex but did not improve the odds of actually centralizing the tip of the implant. CONCLUSIONS AND CLINICAL RELEVANCE: Malpositioning of implants in the sagittal plane may be minimized through ostectomy at the lesser trochanter and use of an undersized implant positioned in normoversion. Use of a polymethylmethacrylate centralizing device will help eliminate contact between the implant tip and adjacent cortex. Implantation of an undersized femoral component, avoidance of substantial anteversion or retroversion, and use of a rigid centralizing device are recommended when using the prosthesis described-for total hip replacement of dogs.     

ULTRASONOGRAPHIC PERCUTANEOUS ANATOMY OF THE CAUDAL LUMBAR REGION AND ULTRASOUND‐GUIDED LUMBAR PUNCTURE IN THE DOG

Subarachnoid lumbar puncture is used commonly in the dog for cerebrospinal fluid collection and/or myelography. Here in we describe the percutaneous ultrasound anatomy of the lumbar region in the dog and a technique for ultrasound‐guided lumbar puncture. Ultrasound images obtained ex vivo and in vivo were compared with anatomic sections and used to identify the landmarks for ultrasound‐guided lumbar puncture. The ultrasound‐guided procedure was established in cadavers and then applied in vivo in eight dogs. The anatomic landmarks for the ultrasound‐guided puncture, which should be identified on the parasagittal oblique ultrasound image are the articular processes of the fifth and sixth lumbar vertebrae and the interarcuate space. The spinal needle is directed under ultrasound‐guidance toward the triangular space located between the contiguous articular processes of the fifth and sixth lumbar vertebrae and then advanced to enter the vertebral canal. Using these precise ultrasound anatomic landmarks, an ultrasound‐guided technique for lumbar puncture is applicable in the dog.     

Revision of Cemented Total Hip Arthroplasty with Cementless Components in Three Dogs

Objective— To report revision of cemented total hip arthroplasty failure with cementless components in 3 dogs.
Study Design— Clinical case reports.
Animals— Dogs with total hip arthroplasty failure (n=3).
Methods— Cementless total hip arthroplasty revision was performed in 3 dogs with previously cemented femoral and acetabular components. All dogs required revision of the femoral component and 1 dog also required revision of the acetabular component.
Results— Revisions resulted in a stable functional prosthesis with successful bone integration.
Conclusions— Use of cementless components may be a viable option for revision of loosened cemented prosthesis after explantation of failed cemented canine total hip replacements.
Clinical Relevance— Failed canine-cemented total hip arthroplasties can be successfully revised with cementless components.     

COMPARISON BETWEEN COMPUTED TOMOGRAPHY,FLUOROSCOPY, AND ULTRASONOGRAPHY FOR GUIDING PERCUTANEOUS INJECTION OF THE CANINE INTERVERTEBRAL DISC

A minimally invasive method for delivering injectable therapeutic agents would be desirable for the treatment of intervertebral disc disease in dogs. The purpose of this study was to compare computed tomography (CT), ultrasonography (US), and fluoroscopy modalities for guiding percutaneous injection into canine intervertebral discs. Intervertebral discs of 14 dog cadavers were injected with a gelified ethanol therapeutic agent. Successful injectate placement and injectate leakage were determined based on necropsy inspection of discs. Injection into the nucleus pulposus was successful in 55 of 78 (71%) of all injected discs. Injections guided using CT and fluoroscopy were significantly more successful than US‐guided injections. Odds of successful injection without leakage were greater for CT vs. US (P = 0.0026) but there was no significant difference between CT and fluoroscopy (P = 0.0620). Injection success rates did not differ among vertebral sites or dog cadavers of varying weights. Forty‐nine (63%) of injection sites had injectate leakage outside the disc and 10 of these involved structures within the vertebral canal. The highest rate of injection success with the least amount of leakage was achieved with CT guidance. Findings indicated that CT, fluoroscopy, and US are feasible modalities for guiding percutaneous injection of a gelified ethanol therapeutic agent into the canine intervertebral disc, with moderate to high success rates for different regions of the spine. However, a moderately high rate of injectate leakage occurred outside of the disc and this should be taken into consideration for future safety and efficacy studies.     

Short-term evaluation of dorsal acetabular augmentation in 10 canine total hip replacements

OBJECTIVE: To report the use of dorsal acetabular augmentation (DAA) in canine total hip arthroplasty (THA) and to evaluate the clinical and radiographic outcome after a minimum of 1 year follow-up. STUDY DESIGN: Clinical study. ANIMALS: Nine dogs that had THA. METHODS: Ten hips requiring acetabular augmentation for optimum acetabular cup implantation for THA were evaluated retrospectively. The excised femoral head and neck was used as the corticocancellous bone graft for index THA surgeries, and the dorsal crest of the ipsilateral ilial wing was used as the bone graft in 1 dog that had a revision surgery. Clinical and radiographic follow-up examinations were performed at approximately 8 weeks and 1 year postoperatively. Long-term radiographic examinations included use of a standing dorsal acetabular rim (DAR) projection for assessment of graft-recipient boundary. Functional clinical outcome was assessed using direct patient evaluation and telephone interview. RESULTS: One dog failed to regain limb function after surgery. Suspected aseptic loosening of the acetabular cup was observed on radiographs approximately 12 weeks postoperatively. Excluding this single explantation, follow-up physical and radiographic examinations indicated good implant and graft positioning and normal function for all dogs. CONCLUSIONS: Acetabular augmentation permitted implantation of an appropriately sized acetabular cup while maintaining complete DAR coverage of the prosthesis. Nine out of 10 hips had a successful outcome with minimal radiographic and no functional abnormalities. Further follow-up is necessary to determine the long-term outcome of DAR augmentation in canine THA. CLINICAL RELEVANCE: Acetabular augmentation with an autogenous corticocancellous bone graft constructed from the femoral head and neck can provide complete DAR coverage to permit use of an appropriately sized acetabular cup in THA.     

Modular hybrid total hip arthroplasty. Experimental study in dogs

Background

This prospective experimental study evaluated the surgical procedure and results of modular hybrid total hip arthroplasty in dogs.

Methods

Ten skeletally mature healthy mongrel dogs with weights varying between 19 and 27 kg were used. Cemented modular femoral stems and uncemented porous-coated acetabular cups were employed. Clinical and radiographic evaluations were performed before surgery and at 30, 60, 90, 120, 180 and 360 days post-operation.

Results

Excellent weight bearing was noticed in the operated limb in seven dogs. Dislocation followed by loosening of the prosthesis was noticed in two dogs, which were therefore properly treated with a femoral head osteotomy. Femoral fracture occurred in one dog, which was promptly treated with full implant removal and femoral osteosynthesis.

Conclusions

The canine modular hybrid total hip arthroplasty provided excellent functionality of the operated limb.     

The Effect of Triple Pelvic Osteotomy on the Articular Contact Area of the Hip Joint in Dysplastic Dogs: An in Vitro Experimental Study

Objective —To investigate the effect of triple pelvic osteotomy (TPO) on articular contact area and acetabular coverage of dysplastic hip joints in dogs.
Study Design —Articular contact area and femoral head coverage by the acetabulum were computed in vitro in normal and dysplastic canine hips. The effect of TPO on articular contact and coverage was then analyzed in the dysplastic hips.
Sample Population—Five normal and six dysplastic canine cadaver specimens.
Methods —Contact area and coverage of loaded hips were computed using serial computed tomography scan images before and after TPO. Three angles of acetabular ventroversion (AVV) were studied (20°, 30°, and 40°). Using a custom-designed hinge plate, angles of spontaneous hip reduction in dysplastic hips were compared with previously recorded angles of reduction determined by the Ortolani test.
Results —Contact area significantly increased from 0° to 30° of AVV, then remained virtually unchanged. Coverage significantly increased from 0° to 20° of AVV. Both contact and coverage of normal hips were similar, yet significantly smaller than those of dysplastic hips once reduction had occurred. The experimental angles of reduction were significantly smaller and poorly correlated with the angles of reduction determined by the Ortolani test. Although coverage continued to increase with AVV, the actual joint contact area did not significantly vary after relocation of the femoral head.
Conclusions —This study suggests that increasing AVV beyond 20° does not significantly improve the beneficial effects of TPO and therefore should be carefully weighed against increased risks of postoperative complications associated with large angles of AVV.     

Morphometric assessment of the canine hip joint using the dorsal acetabular rim view and the centre-edge angle

The dorsal acetabular rim (DAR) view of the hip joint can be used to assess the weightbearing portion of the acetabulum and the acetabular coverage, providing an adjunct to the conventional ventrodorsal (VD) view in the radiographic evaluation of hip dysplasia in the dog. A quantitative index of acetabular coverage in the DAR view, the acetabular slope (AS) angle, was originally proposed in 1990. The aim of the present study was to make a prospective, comparative assessment of a new parameter, the centre-edge (CE) angle, with the AS angle, for the evaluation of the acetabular coverage of the femoral head in the DAR view. The reliability and repeatability of the two parameters was assessed using the r(I) value of intraclass correlation coefficient (ICC) in a prospective study of 208 hip joints in large and giant breed dogs. An estimation of the added value of using the DAR view, compared with that of the VD standard view alone, was also assessed. The CE angle showed a higher r(I) value compared with the AS angle; in 26 per cent of hips of FCI classes A, B and C, the DAR view provided additional diagnostic information compared with the VD view, with respect to lateralisation and/or initial changes to the dorsal rim. It is concluded that the CE angle is more reliable than the AS angle in the evaluation of acetabular coverage, and that the DAR view provides valuable data compared with the VD view alone in the early stages of canine hip dysplasia.     

TECHNIQUE,DIFFICULTY, AND ACCURACY OF COMPUTED TOMOGRAPHY‐GUIDED TRANSLAMINAR AND TRANSFORAMINAL LUMBOSACRAL EPIDURAL AND INTRAARTICULAR LUMBAR FACET JOINT INJECTIONS IN DOGS

In human medicine, spinal pain and radiculopathy are commonly managed by computed tomography (CT)‐guided facet joint injections and by transforaminal or translaminar epidural injections. In dogs, CT‐guided lumbosacral epidural or lumbar facet joint injections have not been described. The aim of this experimental, ex vivo, feasibility study was to develop techniques and to assess their difficulty and accuracy. Two canine cadavers were used to establish the techniques and eight cadavers to assess difficulty and accuracy. Contrast medium was injected and a CT scan was performed after each injection. Accuracy was assessed according to epidural or joint space contrast opacification. Difficulty was classified as easy, moderately difficult, or difficult, based on the number of CT scans needed to guide insertion of the needle. A total of six translaminar and five transforaminal epidural and 53 joint injections were performed. Translaminar injections had a high success rate (100%), were highly accurate (75%), and easy to perform (100%). Transforaminal injections had an moderately high success rate (75%), were accurate (75%), and moderately difficult to perform (100%). Success rate of facet joint injections was 62% and was higher for larger facet joints, such as L7‐S1. Accuracy of facet joint injections ranged from accurate (37–62%) to highly accurate (25%) depending on the volume injected. In 77% of cases, injections were moderately difficult to perform. Possible complications of epidural and facet joint injections were subarachnoid and vertebral venous plexus puncture and periarticular spread, respectively. Further studies are suggested to evaluate in vivo feasibility and safety of these techniques.     

Relationships among measurements obtained by use of computed tomography and radiography and scores of cartilage microdamage in hip joints with moderate to severe joint laxity of adult dogs

OBJECTIVE: To evaluate correlations among measurements on radiographic and computed tomography (CT) images with articular cartilage microdamage in lax hip joints of dogs. ANIMALS: 12 adult mixed-breed hounds. PROCEDURES: Pelvic CT and radiography were performed. Hip joints were harvested following euthanasia. Orthopedic Foundation for Animals (OFA) and PennHIP radiograph reports were obtained. Norberg angle (NA) and radiographic percentage femoral head coverage (RPC) were determined. Center-edge angle (CEA), horizontal toit externe angle (HTEA), ventral acetabular sector angle (VASA), dorsal acetabular sector angle (DASA), horizontal acetabular sector angle (HASA), acetabular index (AI), and CT percentage femoral head coverage (CPC) were measured on 2-dimensional CT images. Femoral head-acetabular shelf percentage was measured on sagittal 3-dimensional CT (SCT) and transverse 3-dimensional CT (TCT) images. Light microscopy was used to score joint cartilage. Relationships of OFA confirmation and PennHIP osteoarthritis scores with radiography, CT, and cartilage variables and relationships of cartilage scores with radiography and CT measurements were evaluated with Spearman rank correlations. Pearson correlation was used for relationships of distraction index (DI) with radiography, CT, and cartilage variables. RESULTS: Significant relationships included PennHIP osteoarthritis score with cartilage score, CEA, HTEA, DASA, AI, CPC, and TCT; OFA confirmation score with cartilage score, NA, RPC, CEA, HTEA, DASA, AI, CPC, and TCT; cartilage score with NA, RPC, CEA, HTEA, DASA, HASA, AI, and TCT; and DI with cartilage score, CEA, HTEA, DASA, HASA, AI, and CPC. CONCLUSIONS AND CLINICAL RELEVANCE: CT appeared to be a valuable imaging modality for predicting cartilage microdamage in canine hip joints.     

Retrospective analysis of canine miniature total hip prostheses

OBJECTIVE: To determine the practicality and clinical outcome of miniature total hip replacement (THR) in dogs. STUDY DESIGN: Retrospective study. Sample Population- Seventeen client-owned dogs that had miniature THR. METHODS: Patient data surveyed included signalment, body weight, diagnosis, implant size, surgical technique, and intraoperative and postoperative complications. Radiographic evaluation included angle of lateral opening of the acetabular component, implant positioning, cement mantle quality, and femoral displacement measurement and ratio. Client questionnaire and orthopedic examination were used to obtain long-term follow-up information. RESULTS: Miniature THR was performed to address hip dysplasia and secondary osteoarthritis. In 1 dog, a staged bilateral procedure was performed. Mean body weight was 19 kg (range, 12 to 25 kg). Penetration or fissure of the femoral cortex was the most common intraoperative complication and occurred in 3 dogs. In 3 dogs, there was excessive coxofemoral laxity after reduction of the prosthesis. This instability was addressed specifically in 2 dogs by capsulorrhaphy or capsular prosthesis. Postoperative convalescent complications (craniodorsal luxation, 2 dogs; acetabular cup displacement, 1 dog) were related to surgical errors. Aseptic loosening of the femoral implant was diagnosed in 1 dog at 18 months. Mean follow-up time was 17 months (range, 4 to 42 months). Fifteen of 18 (83%) miniature THRs had good or excellent outcomes. CONCLUSIONS: Miniature THR should be considered a satisfactory alternative to femoral head and neck ostectomy in medium-size dogs affected by hip dysplasia and secondary osteoarthritis. The population of medium-size dogs that might derive more benefit from THR than FHO has yet to be defined. CLINICAL RELEVANCE: Miniature THR is a viable treatment option in medium-size dogs with hip dysplasia.     

Ultrasonographic Percutaneous Anatomy of the Atlanto‐Occipital Region and Indirect Ultrasound‐Guided Cisternal Puncture in the Dog and the Cat

Cisternal puncture in dogs and cats is commonly carried out. This article describes the percutaneous ultrasound anatomy of the cisternal region in the dog and the cat and an indirect technique for ultrasound‐guided cisternal puncture. Ultrasound images obtained ex vivo and in vivo were compared with anatomic sections and used to identify the landmarks for ultrasound‐guided cisternal puncture. The ultrasound‐guided procedure was established in cadavers and then applied in vivo in seven dogs and two cats. The anatomic landmarks for the ultrasound‐guided puncture are the cisterna magna, the spinal cord, the two occipital condyles on transverse images, the external occipital crest and the dorsal arch of the first cervical vertebra on longitudinal images. Using these ultrasound anatomic landmarks, an indirect ultrasound‐guided technique for cisternal puncture is applicable in the dog and the cat.     

Pelvic osteotomy for axial rotation of the acetabular segment in dogs with hip dysplasia.

Pelvic osteotomy has provided normal function and activity to dogs with hip dysplasia. Physical and radiographic examination and palpation under anesthesia permit evaluation of the degenerative process and provide the specific amount of axial rotation of the pelvis necessary for each dog. The technique consists of an osteotomy of the pubis, ischium, and ilium to reestablish acetabular support of the femoral head.