Doctors

David Prybyla, MD

Dr David Prybyla | Orthopaedic Surgeon Chelmsford MADr. Prybyla is a graduate of Yale University and Albany Medical College. He completed his residency training at the University of Massachusetts Medical School in Worcester, followed by subspecialty training in adult joint reconstruction surgery at the New England Baptist Hospital in Boston. Dr. Prybyla is interested in adult joint replacement surgery and trauma.


Christopher Baker, MD

Dr Christopher Baker | Orthopaedic Surgeon Chelmsford MADr. Baker is a graduate of the University of Michigan and Boston University School of Medicine. He completed his residency training at the University of Massachusetts Medical School in Worcester. He also completed subspecialty training in adult Foot and Ankle surgery at the New England Baptist Hospital in Boston.  Dr. Baker holds privileges at Lowell General Hospital.

Dr. Baker is board certified by the American Board of Orthopedic surgeons and is a member of the American Orthopedic Foot and Ankle Society. Dr. Baker is interested in adult foot and ankle surgery and trauma.


Elisabeth Gennis, M.D.

Dr David Prybyla | Orthopaedic Surgeon Chelmsford MADr. Elisabeth Gennis completed her undergraduate degree at Cornell University in Ithaca, NY and a Masters of Public Health at New York University in New York City. She subsequently graduated with a medical degree from the University of Miami Miller School of Medicine in Miami, FL. She completed her orthopaedic residency training at Boston Medical Center in Boston, MA and went on to obtain fellowship training in the hand and upper extremity at Jackson Memorial Hospital in Miami, FL. Her areas of expertise include conditions of the hand and upper extremity such as arthritis, trauma, compressive neuropathies, tendinopathies and deformity in both children and adults.

Dr. Gennis is board-eligible by the American Board of Orthopaedic Surgery. She has privileges at Lowell General Hospital, Emerson Hospital, Mount Auburn Hospital and Nashoba Valley Medical Center.


Physician Assistants

Allison Devine, PA-C, ATC

Allison is a board certified physician assistant as well as a board certified athletic trainer. She works with Dr. Wu providing patient care for a broad range of orthopedic injuries including sports medicine, total joint replacements, and fracture care. She also first assists in the operating room.

Allison graduated MCPHS University - Manchester with a masters degree in physician assistant studies in 2016. She is also a 2013 graduate of Northeastern University with a bachelors of science in athletic training. Prior to attending graduate school, she worked as a physician extender and clinical support staff for an orthopedic surgical practice in southern NH.

She is a member of the American Academy of Physician Assistants and National Athletic Training Association.


DJ Tibbetts, MPAS, PA-C, ATC, LAT

DJ Tibbetts is a Board Certified Physician Assistant who has worked directly with Dr. Bhat since January 2017. DJ assists Dr. Bhat in providing orthopedic care to patients with spinal disorders.

DJ is a graduate of Massachusetts College of Pharmacy and Health Science with a Master’s of Physician Assistant Study degree and has been certified and licensed as a PA since 2015.

With his added qualifications as a Certified and Licensed Athletic Trainer, DJ served as the Head Athletic Trainer and Assistant Athletic Director at Austin Preparatory School in Reading MA prior to becoming a PA from 2009-2011.


Nurse Practitioners

Roula Johnstone, NP-C

Roula Johnstone is a Board Certified Acute Care Nurse Practitioner who has worked directly with Dr. Alter since 2011. Roula assists Dr. Alter in providing orthopedic care to patients with upper extremity disorders.

Roula is a graduate of Northeastern University with a Master's of Science degree in Nursing, where she also obtained her Acute Care Nurse Practitioner certification in 2011. She obtained her Bachelors Degree of science in Nursing from UMASS Lowell in 1999. Roula also obtained her Registered Nurse First Assistant certificate from Delaware County Community College in 2006, and has since been assisting in surgery.

Prior to her masters degree, she worked as a critical care nurse and registered nurse first assistant at Saints Medical Center in Lowell.

She is a member of the American Academy of Nurse Practitioners, the Massachusetts Coalition of Nurse Practitioners, and the Association of Operating Room Nurses.


Kristen Proverb, NP-C

Kristen Proverb is a certified nurse practitioner who has provided specialized orthopaedic care to patients of Dr. Scott Sigman since 2006. At Orthopaedic Surgical Associates, Kristen assists in the diagnosis and treatment of a wide range of orthopaedic conditions, focusing primarily on sports medicine. She has experience with x-ray, MRI, CT and other imaging exams, as well as pre- and post-operative care.

Kristen graduated with her Bachelor’s degree in Political Science at the University of Massachusetts before completing a Master of Science degree in Nursing at Northeastern University as part of the Direct Entry ANP Program. Upon graduation, Kristen was certified as an Adult Nurse Practitioner. Kristen served as a clinical Nurse at Massachusetts General Hospital specializing in vascular surgery and critical care at North Shore Medical Center.

Before joining Dr. Sigman’s team, Kristen served as a Clinical Nurse at Massachusetts General Hospital and North Shore Medical Center. She is a member of the American Academy of Nurse Practitioners and the Massachusetts Coalition of Nurse Practitioners.


June Stott, APRN, BC, MS, CRNFA


Jessica Carey, NP-C


Caitlin Mahoney ACNPC-AG

Caitlin Mahoney is a Certified Acute Care Nurse Practitioner who graduated in 2017. She works directly with Dr. Scott Sigman providing comprehensive assessment, pre-operative, operative and post-operative care across a variety of orthopedic conditions, specializing in sports medicine. She is experienced in reading and interpreting diagnostic imaging, including X-ray, MRI, and CT scan.

Caitlin earned her undergraduate Bachelors degree in Biology/Pre-Med at Assumption College. She is also a graduate of Massachusetts General Hospital Institute of Health Professions where she earned a Bachelors of Science in Nursing and a Masters of Science in Nursing. She obtained her Acute Care Nurse Practitioner certification in 2017.

She also earned a Certificate of Advanced Study in Global Health in 2016, and was able to provide patient care in an international setting at Kasturba Medical Hospital, Manipal India.

Caitlin has provided nursing care for patients in a variety of settings including the Emergency Department, inpatient respiratory medical floor, and neurosurgery.

Caitlin is an active member of the American Academy of Nurse Practitioners, American Association of Critical Care Nurses and the American Nurses Association.