## 1 Introduction  
Immediate implant placement fails to mitigate post-extraction bone resorption [1]; however, it remains a valuable technique for reducing the number of surgical interventions [2]. Alveolar ridge preservation techniques employing space-filling biomaterials have demonstrated a significant capacity to restrict bone remodeling following extraction and to preserve the alveolar ridge [3; 4; 5; 6; 7]. The deproteinized bovine bone mineral (DBBM) acts as a scaffold for new bone formation [8], exhibiting the ability to enhance soft tissue volume and preserve the alveolar volume when compared to natural healing [9].  
A meta-analysis demonstrated that filling the gap resulted in a 54% decrease in horizontal buccal bone loss and significantly reduced soft tissue displacement [10]. Socket grafting with biomaterial prevents bone resorption with a mean difference of 1.99 mm in horizontal loss, 1.72 mm in vertical mid-buccal, and 1.16 mm in vertical mid-lingual dimensions, compared to tooth extraction alone [7]. Furthermore, alveolar ridge bone resorption in the posterior region diverges from those in the esthetic zone due to anatomical distinctions. Dimensional changes following tooth extraction in the posterior region of humans, when no preservation technique was applied, demonstrated a reduction in bone width of up to 50% during a one-year follow-up, with two-thirds of the total changes occurring within the first three months [11]. Thicker buccal bone and the larger gap fill in the posterior sockets lead to less pronounced ridge alterations [12].  
Insufficient keratinized mucosa (KM) is associated with a higher susceptibility to increase the symptoms of inflammation, such as swelling, erythema, and bleeding on probing [13; 14]. Implant sites with less than 2 mm of keratinized mucosa indicate a higher predisposition for the development of peri-implantitis, as patients demonstrated significantly greater marginal bone reduction, increased brushing sensitivity, and heightened plaque accumulation over a 4-year follow-up period [14].  
Various surgical techniques and materials are employed to enhance soft tissues around dental implants to seal the socket opening, including autogenous tissue such as free gingival graft or sub-epithelial connective tissue graft, barrier membrane, or collagen matrices [2; 7; 15]. However, significant limitations persist in these techniques, including patient morbidity, limited donor tissue availability, infection risk, and the high biomaterial costs [2; 15; 16]. Exposing xenogeneic bone to the oral cavity during immediate implant placement offers a more simplified clinical protocol. However, the scientific literature on this specific technique remains limited [16; 17], warranting further investigation.  
Based on the previous study [16], related to the technique of xenogeneic bone left exposed, the implant gap is covered by a tissue resembling KM during the healing process, and there were no postoperative complications like contamination and rejection or biomaterial loss. Hence, the objective of this study was to investigate immediate dental implant placement in molars sites and to compare the use of collagen matrix and/or xenogeneic bone exposed to the oral environment. The null hypothesis was raised that both treatments are comparable, and there is no significant difference between the amount of KM, dimensional tissue changes, and quality of the healing process.