The goal of this clinical trial is to evaluate the retention and resistance to dislodgment of Bioflx crowns compared to zirconia crowns in the restoration of primary molars. The main question[s] it aims to answer : Is there a difference in the retention and resistance to dislodgement of BioFlx crowns compared to zirconia crowns in the restoration of primary molars?" The study will also assess the clinical performance of both types of crowns in terms of occlusal wear, gingival health, preparation time, as well as child and parental satisfaction.

Primary molars play a significant role in maintaining oral function, esthetics, and space for permanent teeth, therefore, restoring decayed primary molars is a critical aspect of pediatric dentistry. Stainless steel crowns have been the standard restorative material for primary molars due to their clinical success, retention, and durability. However, the aesthetic limitations of SSCs, have been problematic, especially when patients seek more esthetically acceptable treatment options.In an attempt to address the aesthetic limitations of stainless-steel crowns, pre-veneered stainless-steel crowns were introduced featuring a thin layer of composite resin or ceramic on the outside to improve their appearance. While they provided a better aesthetic than pure stainless steel, their limitations included a tendency to wear down over time, difficulty in matching natural tooth color, and the potential for the veneer to chip or detach.Recently, zirconia crowns are preferred over pre-veneered stainless-steel crowns for their esthetic superiority, biocompatibility, and resistance to wear. They demonstrate excellent retention and durability but require significant tooth preparation, which may compromise tooth structure. Despite their advantages, zirconia crowns posed challenges such as increased cost, more complex preparation techniques, and concerns about their brittleness under certain conditions. Moreover, zirconia crowns may not be suitable for all cases, especially those involving less mineralized or structurally compromised primary molars .In 2021, Kids-e-Dental LLP introduced Bioflx Crowns and claimed as the first flexible, durable, and esthetic preformed crown for primary molars. Bioflx Crowns are monochromatic, metal-free, tooth-colored crowns made up of high-strength resin polymer used in the medical device industry having high strength, flexibility, and durability. These crowns are autoclavable and are like SSC in tooth preparation as claimed by the manufacturer. Their flexibility and snap-fit technology aim to improve retention while minimizing trauma during placement.

It is important to mention that retention is a crucial factor for the success of any dental crown. An inadequate retention can lead to crown displacement, necessitating re-treatment, and ultimately affect the long-term health of the primary tooth. While studies have demonstrated the superior esthetic properties of materials like Bioflx crowns, their clinical performance, particularly retention rates and overall longevity, require further evaluation in direct comparison to zirconia crowns .Thus, this trial aims to fill the gap in current knowledge and provide clinicians with evidence to support the choice of crown material based on retention performance, which is critical to the long-term success of pediatric restorations.

Inclusion Criteria:The inclusion criteria:

1. Children aged between 4 and 8 years with primary molar that needs to be covered by crown.
2. Children who can cooperate sufficiently for the duration of the procedure, as assessed by behavior management techniques.
3. Medically healthy children (ASA I or II according to the American Society of Anesthesiologists classification).
4. Children whose parent or guardian are willing to sign an informed consent.
5. Children whose parent or guardian are willing to comply with follow-up visits. -

Exclusion Criteria:

• The exclusion criteria

  1. Molars with severe structural loss that may compromise crown retention.
  2. Children with poor oral hygiene that may contribute to higher plaque levels and affect the clinical outcomes of the crown restorations.
  3. Children with history of allergies; Known allergies to dental local anesthesia or to the materials used in Bioflx crowns.
  4. Children with any systemic conditions or disabilities that may affect their ability to participate in the study.

statistician