About Our Physician Assistants

The trusting and collaborative relationship of the Physician-Physician Assistant (PA) team helps to ensure quality and continuity of patient care. PAs are an essential part of the spine team providing the highest level of surgical and nonsurgical care for patients.

Physician Assistants:


Kristopher Stillwell, RPA-C

Mr. Stillwell is a Board Certified Physician Assistant, who has extensive experience in orthopaedic spine disorders and spine surgery. Kristopher serves as Chief of Mid-level practitioners at Long Island Spine Specialists, P. C.

Mr. Stillwell graduated Cum Laude from St. John’s University, New York in 1999. He began his healthcare career in the ER, and subsequently moved to general orthopaedics. He has been with Long Island Spine Specialists, P.C. since 2001 and has extensive experience in the operating room with complex spinal surgical cases. His passion in caring for people comes through not only at LISS but through his charitable works. He has donated his time and skills with a team, internationally, in performing charitable, life altering, corrective surgeries for children with significant spinal deformities and looks forward to his continued involvement with charitable surgery. He also impacts children’s lives in our community as a volunteer leader with Boy Scouts of America.

In addition to his passion for medicine, Kristopher also serves as Director of Information Technology for Long Island Spine Specialists, P.C. He is a Microsoft Certified System Engineer and has in depth knowledge of health care related IT. Kristopher oversees the integration of EMR and digital x-ray technology. This provides LISS with seamless patient care throughout Nassau and Suffolk counties. His background is a perfect complement to the advanced technologies utilized at LISS.

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Colleen A. Kelly, RPA-C

Ms. Kelly is a Board Certified Physician Assistant whose previous fifteen-year experience as a paramedic and firefighter in Arizona gives her a unique perspective into injuries to the cervical and lumbar spine. She began her Physician Assistant career in physical medicine and rehabilitation practicing in Queens and Brooklyn. Her relocation to Long Island brought her to Long Island Spine Specialists, P.C. where she provides compassionate treatment for orthopaedic spine conditions. Her achievement of Master of Science in Forensic Medicine provides an excellent background with regard to the pathophysiologic basis of traumatic injury and disease.

Ms. Kelly’s passion and commitment is to help her patients return to an active lifestyle. In addition to her work at LISS, Ms. Kelly volunteers at a local animal adoption center for cats and dogs, where she nurses them back to optimal health and adoptability, enabling the animals to find their “forever home”.

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Leah Gustavson, RPA-C

Ms. Gustavson is a Board Certified Physician Assistant. Upon graduation from The State University of New York at Stony Brook in 1994, she practiced Emergency Medicine for three years providing urgent and emergent care to patients and led the Rapid Response Team. She later expanded her role by providing care for inpatient services as well. This experience gives her a holistic perspective on patients through the continuum of care and not just a “diagnosis”. She enjoys working with patients, being a part of the medical team helping to restore function and improve daily living.

Ms. Gustavson’s enthusiastic interest in ecological issues led her to the Hudson River and a position as an educational assistant for a conservational organization, providing environmental education and advocacy. Her volunteer work as an EMT for the Stony Brook University Ambulance Corps for over five years convinced her that Medicine was her life’s work. Volunteerism continues to be part of her commitment to the community. Her desire to provide care and compassion inspired her to join the Suffolk County Medical Reserve Corps serving the Long Island community in times of need or disaster.

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Mohammed W. Sanzar, RPA-C

Mr. Sanzar is a Board Certified Physician Assistant. His volunteer work at Jamaica Hospital, a Level I Trauma Center, presented an introduction to traumatic injuries of the spine, sparking his interest in spine care and impelling him toward his Physician assistant degree and certification.

Upon graduating Cum Laude in 2012 from City University of New York at York College, he launched his medical career in a multi-specialty surgery clinic with a high volume of trauma cases. During his training and work experience, he provided surgical care and post-operative management to patients with diverse surgical conditions involving Orthopaedic, Vascular, Plastic, and General Surgery. He subsequently developed his expertise in managing and treating patients on a comprehensive basis. He prides himself in advancing his spine-specific knowledge and skills for the benefit of the patients he has the privilege to treat. He has built a quality reputation by taking the time to listen to the needs and concerns of each patient.

In addition to his work at Long Island Spine Specialists, P.C., Mr. Sanzar is a member of the Queens Basketball League, a team that raises funds for homeless shelters and other charitable organizations. A perfect complement to the LISS philosophy of giving “Back” to the community.

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