Introduction
The maxillary fourth premolar (P4), commonly referred to as the carnassial tooth, is recognized as the most frequently fractured tooth in dogs. Dental fractures are relatively common in the canine population, with prevalence estimates ranging from 20–27% depending on the cohort studied1,2. Among fractured teeth, the maxillary P4 is consistently reported as the most affected, often accounting for nearly one third of cases3. Its strategic role in mastication, combined with the high occlusal forces it bears, makes this tooth particularly vulnerable to traumatic injury and subsequent complications. A major concern in veterinary dentistry is the delayed detection of such fractures. Uncomplicated crown fractures (UCFs), in which the pulp is not obviously exposed, can remain unnoticed for extended periods. Without timely intervention, these lesions may progress to pulpal necrosis, pulpitis, or secondary endodontic infection, ultimately resulting in periapical pathology. Goodman et al.. reported that 24.3% of maxillary P4 UCFs already had radiographically detectable endodontic lesions at diagnosis4. Traditionally, the presence of periapical abscesses or osteolysis has been considered an indication for extraction. However, recent studies suggest that conservative endodontic therapy may remain a viable alternative even in such cases. Kwon et al.. demonstrated that although preoperative periapical lesions reduced the probability of complete radiographic healing, only 4.16% of teeth with periapical lucencies were ultimately classified as failures after endodontic therapy, highlighting the potential for tooth preservation in selected cases5.

When radiographic evidence of periapical disease is absent, endodontic therapy represents the most appropriate treatment option, preserving the tooth while maintaining function and structural integrity. Retention of the P4 preserves occlusal function, maintains the scissor-like carnassial mechanism, and prevents the anatomical and biomechanical changes associated with tooth loss. Current guidelines, including the WSAVA Global Dental Guidelines, emphasize the importance of thorough clinical and radiographic evaluation to distinguish between teeth requiring extraction and those suitable for endodontic management6. Despite these advantages, endodontic treatment of the maxillary P4 remains technically demanding. The tooth’s complex anatomy, including three roots-mesiobuccal, distobuccal, and palatal-presents challenges for canal location, debridement, and obturation.

Anatomy and Morphological Considerations of the Maxillary Fourth Premolar
The maxillary fourth premolar is a large, three-rooted tooth located immediately caudal to the third premolar and rostral to the first molar. Its elongated, robust crown reflects its primary shearing function. The occlusal surface exhibits a prominent paracone cusp, the principal shearing cusp, accompanied by smaller metacone and protocone cusps. Together, these create a scissor-like occlusion with the mandibular first molar, enabling the carnassial function10.

 The root system consists of three distinct roots: mesiobuccal, distobuccal, and palatal. The distal root is typically the broadest, contributing significantly to the tooth’s resistance to occlusal forces. The palatal root is often the longest when measured from the cemento-enamel junction to the anatomical apex, though this varies with methodology. The mesiobuccal root is smaller, but all three roots combined provide strong alveolar anchorage11,12. Each root contains a canal, and micro-CT studies have demonstrated pronounced curvatures, especially in the mesiopalatal canal, where coronal angulations may exceed 150°, complicating instrumentation10. Access to the palatal canal is further limited by the tooth’s deep maxillary position and surrounding soft tissues10). Morphology is also influenced by breed, size, and age. Larger dogs often have proportionally broader roots and pulp chambers, while smaller breeds have narrower, more tapered canals10,11. Age-related deposition of secondary dentin progressively narrows the pulp chamber, increasing the technical difficulty of instrumentation. In geriatric patients, calcification may obscure canal entrances radiographically13. Radiographically, the P4’s three roots are generally visible, though superimposition can obscure structures. Careful selection of oblique views is required for accurate interpretation, particularly when assessing periapical pathology, resorption, or root fractures14. Certain zones of fragility are consistently observed. The paracone cusp is the largest and most functionally prominent buccal cusp, and because of its size and role in shearing, it is frequently involved in crown fractures. When fractured, the natural fault line often extends toward the pulp chamber, increasing the risk of pulpal exposure or infection. Unfortunately, this pathological opening is not aligned with any of the three root canals, requiring additional preparation to fully access all canals.

Traditional Approaches and Their Limitations
Historically, veterinary endodontic treatment of the maxillary P4 has adapted techniques from human dentistry. The tooth’s three roots, sharp canal curvatures, and narrow diameters, however, limit the effectiveness of conventional methods.

Multi-access approaches
Clinicians have traditionally created multiple coronal access points to reach the mesiobuccal, distobuccal, and palatal canals. Separate vestibular and palatal openings are often needed, particularly when the palatal canal is obscured or at an unfavorable angle. While this improves visualization, it comes at the cost of considerable enamel and dentin removal. The resulting weakening of the crown increases susceptibility to post-treatment fractures of both crown and root7.

Coronal enlargement and structural weakening
Coronal enlargement facilitates straight-line access, a principle borrowed from human dentistry. In the maxillary P4, however, aggressive coronal shaping risks undermining the paracone cusp and thinning buccal root walls. Experimental studies confirm that large or multiple access preparations measurably reduce fracture resistance7,8,9, an important consideration given the high functional loads on canine carnassial teeth.

Instrumentation challenges
Traditional stainless steel hand files are limited in addressing sharp curvatures, increasing the risk of ledging, canal transportation, or perforation. Achieving complete debridement of the apical
third is difficult, particularly in ovoid canals12.

Radiographic limitations
Standard intraoral radiographs provide only 2D views, with overlapping buccal and palatal roots obscuring canal trajectories14. Missed canals are a recognized cause of endodontic failure. Advanced imaging, such as CBCT, improves 3D anatomical understanding but is not yet widely available in routine veterinary practice.

Clinical outcomes
Although traditional techniques can resolve infection and maintain function, structural weakening compromises long-term prognosis. Vertical root fractures, secondary crown fractures, and restorative failure are more likely after multiple or extensive access preparations. These complications highlight the tension between adequate access for thorough therapy and preservation of tooth structure.

Development of the Conservative Single-Access Technique
The single-access technique was developed to balance thorough canal debridement with preservation of crown and root structure.

Conceptual Basis
The cornerstone of this method is a single vestibular access aligned with the natural fracture line at the tip of the paracone cusp. This fracture line is refined into a functional access cavity, permitting negotiation of all three canals using flexible NiTi instruments. Secondary coronal perforations are avoided, preserving dentin that would otherwise be lost.

Access Design
The cavity is initiated on the paracone cusp using a small round diamond bur (ISO 10) to penetrate the pulp chamber. Refinement with a fine tapered diamond bur establishes a triangular outline aligned with the tooth’s longitudinal axis, allowing straight-line access distally toward the distal root and mesially toward the two mesial roots. To improve visualization of the palatal canal, the mesial portion of the access is extended triangularly in the sagittal plane. Enlargement is limited to what is necessary for unobstructed instrumentation.

Instrumentation Strategy
Flexible NiTi rotary files are used to negotiate curved canals. Modern adaptive systems adjust cutting action to canal morphology, enhancing cleaning while minimizing dentin removal. Irrigation protocols with sodium hypochlorite and EDTA complement mechanical debridement.

Advantages
• Minimized tissue removal: Single-point access reduces coronal dentin loss.
• Cusp preservation: Natural fracture line integration spares sound tissue.
• Improved fracture resistance: Limited dentin removal preserves structural integrity.
• Efficient canal management: Flexible instruments navigate complex curvatures without multiple entries.
• Practicality in compromised crowns: Pre-existing fractures are utilized rather than enlarged.

Challenges
• Success depends on precise anatomical knowledge and skilled instrumentation.
• The palatal canal remains the most challenging to access.
• Clinicians must adapt to a smaller, precision-driven access approach.

Conclusion and Future Perspectives
Managing endodontic disease in the canine maxillary P4 is challenging due to its complex anatomy, high functional demands, and frequent fractures. Traditional multi-access techniques, while effective, compromise structural integrity.
The single-access conservative technique offers a practical alternative, leveraging natural fracture lines, flexible instrumentation, and modern irrigation to preserve tooth structure while achieving thorough canal therapy. Preservation of coronal and radicular integrity is critical for long-term function under physiologic loads. Prospective clinical studies comparing single- versus multi-access approaches are needed to quantify differences in fracture incidence, treatment outcomes, and restoration survival. Advanced imaging (CBCT, micro-CT) and evolving instrumentation will likely expand the feasibility of conservative strategies. Adoption within the veterinary dental community will require training programs emphasizing magnification, modern irrigation, and precision instrumentation. In conclusion, single-access endodontics for the canine maxillary P4 represents a significant advancement, balancing biological objectives with biomechanical preservation to extend the functional lifespan of one of the most critical teeth in the canine mouth.

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