An independent clinical audit of dysplasia of the hips (DDH) surgery in children at a number of hospitals has found that a number of the surgeries were unnecessary at Temple Street and Cappagh hospitals.
The review also found a lower threshold was used to decide on surgery at the two hospitals.
At the National Orthopaedic Hospital Cappagh (NOHC), the lower threshold was in 80% of cases and at Children's Health Ireland (CHI) Temple Street it was in 60% of cases.
No such issue was identified at CHI Crumlin.
The report states that while there is worldwide variability between surgeons, the variance identified in this audit cannot be accounted for by measurement error or observer variability alone.
The review looked at 147 cases at the three hospitals and involving 14 surgeons for pelvic osteotomy in children with DDH.
At issue was whether the surgeries were required in all cases.
This type of surgery is where the ball and socket joint of a child's hip has not properly formed.
In the case of the operations reviewed, the bones of the pelvis are cut and reshaped to improve the position of the hip joint.
The audit found that a novel technique was used at Temple Street and Cappagh.
The audit raises questions about the threshold for deciding to use this novel procedure.
The reference to threshold for surgery in the hips report, relates to whether the patient met the international criteria normally required for surgery to be recommended.
An international panel of experts is to be set up and any parents who wish to have their child's case reviewed will be offered this opportunity.
The clinical audit was conducted by consultant paediatric orthopaedic surgeon Mr Simon Thomas, Bristol Royal Hospital for Children in the UK.
Health Service Executive CEO Bernard Gloster said the variance found in surgical practice in relation to developmental dysplasia of the hip surgery is of very serious concern.
He said that the proportion of these surgeries at CHI Temple Street and Cappagh, despite not reaching the international criteria used in the audit, was so high, that the audit report says further inquiry is mandated.
The HSE has said that the group of patients will be contacted directly to explain what this means and the next steps for them.
The HSE said that regrettably the audit identified one case where a child experienced complications due to the surgical approach recommended to them.
The family has been contacted and supported through an open disclosure process, it said.
The audit advises long term follow-up of all children who have pelvic osteotomy surgery should take place and the HSE said teams at CHI and Cappagh are putting plans in place for this.
Mr Gloster also said that a detailed plan has been developed to ensure that in the region of 1,800 children and young adults who have had this surgery at CHI at Temple Street and Cappagh since 2010 are reviewed up to skeletal maturity.
In a statement, Minister for Health Jennifer Carroll McNeill said: "I know that parents and young people themselves will have many questions when they read the report and that the findings of this report will lead to worry and uncertainty for them.
"In responding to this report, I am thinking first and foremost of the impact a surgery has on a child. I am also deeply aware of the worry that parents will face today and this is something that is sitting with me all the time.
"My immediate priority is to ensure that there is clinical follow-up and care for patients who have undergone pelvic osteotomy surgery. This follow-up will be in accordance with best practice and the recommendations of the report."
She said that patients and families will receive letters in the coming days from CHI and NOHC about the arrangements/next steps that will apply to them and their child, and how they can get further information or support.
'I am sorry' - Children's Health Ireland CEO
In a statement, CHI CEO Lucy Nugent said it fully accepts the findings and recommendations from the audit.
She said: "The care and wellbeing of children is our absolute priority in Children's Health Ireland, and I am sorry that impacted families were not offered one consistent and excellent standard of care across our DDH service.
"I do not underestimate the stress and anxiety that families are rightfully feeling throughout this audit process.
"To one family in particular, whose child experienced complications during their care in Children's Health Ireland, I extend my heartfelt apology that we have let you down.
"I would like to give my assurance to all of the families affected, that they will be directly supported and are being contacted with follow up information and the next steps.
"We are currently standardising care across all CHI sites, so all children receive the same high-quality treatment no matter where they are seen. Also, it is important that we now act swiftly to review DDH surgery patients to enable us to answer outstanding questions raised by this audit. This will be done openly and transparently."
CEO of NHOC Angela Lee said: "The National Orthopaedic Hospital Cappagh apologises for the distress that today's published report may understandably cause to children who have had surgery in NOHC and to their parents.
"In recent years, Cappagh has worked with Children's Health Ireland to improve children's access to planned paediatric orthopaedic surgery.
"We are committed to implementing the audit recommendations. Some changes are already under way, with others planned and we will be communicating with all parents/guardians accordingly in the coming days."
The Cathaoirleach of the Oireachtas Committee on Health, Social Democrats TD Pádraig Rice, has issued an invite to the CHI to appear before the committee to discuss the "deeply troubling report".
Mr Rice said although the Minister for Health has asked for time in the Dáil next week to discuss the audit, this should be a questions and answers session rather than a statement.
'Very concerned' and 'deeply disturbing' - Taoiseach
Taoiseach Micheál Martin said he is "very concerned" about the findings of the audit.
Opposition parties have called for a wider review to now take place into what happened.
Speaking in Cork, the Taoiseach said the audit's findings are "very serious" and that he is "concerned" about what has taken place.
Hi "first reaction is with the parents" of the children involved and that they will receive individual clinical follow-ups.
The Taoiseach said the review has found issues over clinical performance and clinical governance at CHI hospitals which must be addressed, adding a widening of the clinical review is likely.
Watch: Taoiseach says there is a need to 'widen the clinical inquiry'
Sinn Féin's Spokesperson on Health David Cullinane described the review as "deeply disturbing" and said parents want to know if their child needed the procedure or not.
He said while an international expert review panel will be set up which parents can contact, it has yet to be established, which he said means some parents may have to wait months more for answers.
Social Democrats TD Sinéad Gibney said she was thinking of the families today who are impacted by this.
She said that her party colleague, Pádraig Rice, would be calling on the Minister for Health to answer questions on it and would also write to CHI to come before the Oireachtas Committee on Health because there are questions to answer about governance.
On patients who are expected to be called for review, she said that this needed to be independently done and that a timeline on this was needed.
People Before Profit-Solidarity TD Paul Murphy described the audit's findings as "a national scandal" and said all hip surgeries at Temple Street and Cappagh hospitals should now be examined.
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